/* ============================================================
   VasmuStudy — Common CSS for Biology Chapters
   Shared styles for Chapters 1–16
   ============================================================ */

/* ============================================================
   1. RESET & BASE
   ============================================================ */
*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  scroll-behavior: smooth;
  scroll-padding-top: 80px;
}

body {
  font-family: var(--body);
  background: var(--bg);
  color: var(--ink);
  line-height: 1.75;
  font-size: 17px;
  -webkit-font-smoothing: antialiased;
  overflow-x: hidden;
  transition: background 0.3s, color 0.3s;
}

a { color: inherit; }
img { max-width: 100%; height: auto; }


html.dark .answer-body {
  background: #0f2318;
  color: #cbd5e1;
  border-top-color: rgba(34, 197, 94, 0.3);
}
html.dark .tf-correction { background: #0f2318; }
html.dark .tf-correction strong { color: #cbd5e1; }
html.dark .image-frame { background: #0f2318; }
html.dark .diagram-img  { background: #14291c; }
html.dark .answer-card:hover { box-shadow: 0 18px 48px rgba(0,0,0,.6); }
html.dark .answer-no { color: #86efac; }
html.dark .marks-total { background: rgba(34,197,94,.15); color: #86efac; }

/* ============================================================
   2. CSS VARIABLES — MAIN THEME (Chapters 1–13)
   ============================================================ */
:root {
  --bg: #f8f9fc;
  --bg-alt: #eef1f8;
  --alt: #eef1f8;
  --card: #ffffff;
  --ink: #0f172a;
  --muted: #64748b;
  --green: #059669;
  --green-d: #065f46;
  --green-l: #10b981;
  --cyan: #0891b2;
  --gold: #d97706;
  --red: #dc2626;
  --purple: #7c3aed;
  --pink: #db2777;
  --border: rgba(15, 23, 42, 0.1);
  --shadow-sm: 0 1px 2px rgba(15, 23, 42, 0.06), 0 2px 8px rgba(15, 23, 42, 0.05);
  --shadow-md: 0 4px 12px rgba(15, 23, 42, 0.08), 0 12px 32px rgba(15, 23, 42, 0.07);
  --shadow-lg: 0 18px 48px rgba(15, 23, 42, 0.14), 0 6px 16px rgba(15, 23, 42, 0.08);
  --serif: "Playfair Display", Georgia, serif;
  --body: "Lora", Georgia, serif;
  --ui: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  --mono: "JetBrains Mono", monospace;
}

/* ============================================================
   3. LEGACY THEME OVERRIDES (Chapters 14–16)
   ============================================================ */
.legacy-theme {
  --bg: #f7faf7;
  --bg-alt: #eaf5ee;
  --alt: #eaf5ee;
  --card: #ffffff;
  --ink: #17251d;
  --muted: #5b6b60;
  --green: #176b45;
  --green-d: #0f4c30;
  --green-l: #22c55e;
  --emerald: #059669;
  --teal: #0d9488;
  --lime: #65a30d;
  --forest: #14532d;
  --border: rgba(23, 107, 69, 0.14);
  --shadow-sm: 0 1px 2px rgba(23, 37, 29, 0.06), 0 2px 8px rgba(23, 107, 69, 0.06);
  --shadow-md: 0 4px 12px rgba(23, 37, 29, 0.08), 0 12px 32px rgba(23, 107, 69, 0.07);
  --shadow-lg: 0 18px 48px rgba(23, 37, 29, 0.14), 0 6px 16px rgba(23, 107, 69, 0.08);
}

/* ============================================================
   4. DARK MODE — legacy theme only
   ============================================================ */
html.dark {
  --bg: #0a1810;
  --bg-alt: #0f2318;
  --alt: #0f2318;
  --card: #14291c;
  --ink: #e0f2e6;
  --muted: #94a89c;
  --border: rgba(148, 168, 156, 0.15);
  --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.4), 0 2px 8px rgba(0, 0, 0, 0.3);
  --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.5), 0 12px 32px rgba(0, 0, 0, 0.4);
  --shadow-lg: 0 18px 48px rgba(0, 0, 0, 0.6), 0 6px 16px rgba(0, 0, 0, 0.5);
}

html.dark .viz { background: #f8f5ff; }
html.dark .def { background: linear-gradient(135deg, rgba(22, 163, 74, 0.12), rgba(13, 148, 136, 0.08)); }
html.dark .note { background: rgba(13, 148, 136, 0.12); }
html.dark .real { background: linear-gradient(135deg, #2a2308, #3a2e0a); border-left-color: #d97706; }
html.dark .real p, html.dark .real ul, html.dark .real li { color: #fcd34d; }
html.dark .real i { color: #fbbf24; }
html.dark .mistake { background: linear-gradient(135deg, #3a0d0d, #4a1414); border-left-color: #dc2626; }
html.dark .mistake p { color: #fca5a5; }
html.dark .mistake i { color: #f87171; }
html.dark .why { background: linear-gradient(135deg, #0f2318, #14291c); border-left-color: #22c55e; }
html.dark .why p { color: #86efac; }
html.dark .analogy { background: linear-gradient(135deg, #1e0f3d, #26144f); border-left-color: #7c3aed; }
html.dark .analogy p { color: #c4b5fd; }
html.dark .exam { background: linear-gradient(135deg, #0f2318, #14291c); color: #86efac; }
html.dark .trap { background: linear-gradient(135deg, #3a1d08, #4a2610); border-left-color: #ea580c; }
html.dark .trap p { color: #fdba74; }
html.dark .try { background: linear-gradient(135deg, #082a1d, #0a3527); border-left-color: #059669; }
html.dark .try p, html.dark .try li { color: #6ee7b7; }
html.dark .step { background: #0f2318; }
html.dark .step p { color: #cbd5e1; }
html.dark .sa { background: #14291c; border-color: rgba(34, 197, 94, 0.3); }
html.dark .sa button { background: rgba(34, 197, 94, 0.15); color: #86efac; }
html.dark .sa .b { border-top-color: rgba(34, 197, 94, 0.3); }
html.dark .o label { background: #0f2318; color: #cbd5e1; }
html.dark .o label:hover { background: #1a3d28; }
html.dark .ex { background: rgba(34, 197, 94, 0.12); color: #cbd5e1; }
html.dark table, html.dark td { color: #cbd5e1; }
html.dark td:first-child { color: #bbf7d0; }
html.dark .interactive { background: #0f2318; border-color: rgba(34, 197, 94, 0.3); }
html.dark .interactive h4 { color: #86efac; }
html.dark .calc-input { background: #1a3d28; color: #d1fae5; border-color: rgba(34, 197, 94, 0.3); }
html.dark select { background: #1a3d28; color: #cbd5e1; border-color: rgba(34, 197, 94, 0.3); }
html.dark .calc-results { background: linear-gradient(135deg, #0f2318, #14291c); }
html.dark .calc-results .res-item { background: #1a3d28; border-color: rgba(34, 197, 94, 0.3); }
html.dark .calc-results .lbl { color: #86efac; }
html.dark .calc-results .val { color: #4ade80; }
html.dark .formula-sheet { background: #14291c; }
html.dark .formula-item { background: #0f2318; border-color: rgba(34, 197, 94, 0.2); }
html.dark .formula-item .ftitle { color: #86efac; }
html.dark .takeaway { background: linear-gradient(135deg, #0f2318, #14291c); border-left-color: #22c55e; }
html.dark .takeaway p { color: #86efac; }
html.dark .notation { background: #0f2318; }
html.dark .notation td { color: #cbd5e1; }
html.dark .flow-node { fill: #14291c; stroke: #22c55e; }
html.dark .flow-node-text { fill: #86efac; }
html.dark .flow-arrow { stroke: #22c55e; }
html.dark .ncert-q { background: #14291c; border-color: rgba(34, 197, 94, 0.25); }
html.dark .summary-box { background: #0f2318; border-color: rgba(34, 197, 94, 0.3); }
html.dark .summary-box li { color: #cbd5e1; }
html.dark .exam-q-box { background: #0f2318; border-color: rgba(34, 197, 94, 0.3); }
html.dark .mnemonic-box { background: #0f2318; border-color: rgba(34, 197, 94, 0.3); }
html.dark .flashcard-static { background: #14291c; border-color: rgba(34, 197, 94, 0.3); }
html.dark .flashcard-static .fc-title { color: #86efac; }
html.dark .flashcard-static .fc-formula { color: #4ade80; background: linear-gradient(135deg, rgba(34, 197, 94, 0.15), rgba(13, 148, 136, 0.08)); }
html.dark .related-links a { background: #14291c; border-color: rgba(34, 197, 94, 0.3); color: #86efac; }
html.dark .chapter-nav a { background: #14291c; border-color: rgba(34, 197, 94, 0.3); color: #86efac; }
html.dark .constants-table { background: #0f2318; }
html.dark .constants-table td { color: #cbd5e1; }
html.dark .read-btn { background: #14291c; border-color: rgba(34, 197, 94, 0.3); color: #86efac; }
html.dark .read-btn.done { background: rgba(16, 185, 129, 0.15); border-color: rgba(16, 185, 129, 0.4); color: #6ee7b7; }
html.dark .did-you-know { background: linear-gradient(135deg, #0f2318, #1a3d28); border-color: rgba(34, 197, 94, 0.3); }
html.dark .did-you-know p { color: #86efac; }
html.dark .rb { background: rgba(34, 197, 94, 0.15); color: #86efac; }
html.dark .nav { background: rgba(10, 24, 16, 0.94); }

/* ===== FIX: Hero background in dark mode (Chapters 14–16) ===== */
html.dark .hero {
  background:
    radial-gradient(900px 500px at 10% -10%, rgba(5, 150, 105, 0.12), transparent 62%),
    radial-gradient(800px 480px at 95% 10%, rgba(8, 145, 178, 0.10), transparent 60%),
    linear-gradient(180deg, var(--card) 0%, var(--bg-alt) 100%);
}

html.dark .hero h1 { color: var(--ink); }
html.dark .hero .lede { color: var(--muted); }
html.dark .hero .eyebrow {
  color: #86efac;
  background: rgba(34, 197, 94, 0.12);
  border-color: rgba(34, 197, 94, 0.25);
}
html.dark .hero .meta-chip {
  background: rgba(20, 41, 28, 0.85);
  color: var(--ink);
  border-color: rgba(34, 197, 94, 0.25);
}
html.dark .meta-info span { color: var(--muted); }

/* ============================================================
   5. PROGRESS BAR
   ============================================================ */
.progress-track,
.pt {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: transparent;
  z-index: 999;
}

.progress-fill,
#progress {
  height: 100%;
  width: 0%;
  background: linear-gradient(90deg, var(--green), var(--cyan));
  box-shadow: 0 0 12px rgba(5, 150, 105, 0.5);
  transition: width 0.1s linear;
}

/* ============================================================
   6. NAVIGATION
   ============================================================ */
.nav {
  position: sticky;
  top: 0;
  z-index: 900;
  background: rgba(248, 249, 252, 0.94);
  backdrop-filter: blur(14px);
  -webkit-backdrop-filter: blur(14px);
  border-bottom: 1px solid var(--border);
  transition: background 0.3s;
}

.nav-inner,
.ni {
  max-width: 1240px;
  margin: 0 auto;
  padding: 10px 24px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 18px;
}

.brand {
  display: flex;
  align-items: center;
  gap: 12px;
  text-decoration: none;
  flex: none;
}

.brand-logo,
.logo,
.f-logo {
  width: 42px;
  height: 42px;
  display: grid;
  place-items: center;
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  font-family: var(--serif);
  font-size: 20px;
  font-weight: 800;
  border-radius: 11px;
  flex: none;
  box-shadow: 0 4px 14px rgba(5, 150, 105, 0.35);
  object-fit: contain;
}

.brand-words {
  display: flex;
  flex-direction: column;
  line-height: 1.05;
}

.brand-name,
.bn {
  font-family: var(--ui);
  font-weight: 800;
  font-size: 20px;
  letter-spacing: -0.02em;
  color: var(--ink);
}

.brand-name .accent,
.bn .accent,
.bn b {
  color: var(--green);
}

.brand-tag {
  font-family: var(--ui);
  font-size: 9.5px;
  font-weight: 600;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--muted);
  margin-top: 2px;
}

.nav-links,
.nl {
  display: flex;
  gap: 2px;
  flex-wrap: nowrap;
  overflow-x: auto;
  scrollbar-width: none;
  align-items: center;
}

.nav-links::-webkit-scrollbar,
.nl::-webkit-scrollbar { display: none; }

.nav-links a,
.nl a {
  font-family: var(--ui);
  font-size: 13px;
  font-weight: 500;
  color: var(--muted);
  text-decoration: none;
  white-space: nowrap;
  padding: 7px 12px;
  border-radius: 999px;
  transition: all 0.22s ease;
  position: relative;
}

.nav-links a:hover,
.nl a:hover {
  color: var(--green);
  background: rgba(5, 150, 105, 0.07);
}

.nl a.read::after {
  content: "✓";
  position: absolute;
  top: 2px;
  right: 2px;
  font-size: 9px;
  color: var(--green);
  font-weight: 900;
}

.theme-btn,
.print-btn {
  background: transparent;
  border: 1.5px solid var(--border);
  border-radius: 999px;
  padding: 8px 12px;
  cursor: pointer;
  font-size: 15px;
  color: var(--ink);
  transition: 0.2s;
  margin-left: 8px;
  font-family: var(--ui);
}

.print-btn {
  font-size: 13px;
  font-weight: 600;
  padding: 8px 14px;
  margin-left: 4px;
}

.theme-btn:hover,
.print-btn:hover {
  border-color: var(--green);
  color: var(--green);
}

/* ============================================================
   7. BREADCRUMB
   ============================================================ */
.breadcrumb,
.bc {
  max-width: 1120px;
  margin: 0 auto;
  padding: 20px 24px 0;
  font-family: var(--ui);
  font-size: 13px;
  color: var(--muted);
  display: flex;
  gap: 8px;
  align-items: center;
  flex-wrap: wrap;
}

.breadcrumb a,
.bc a {
  color: var(--green);
  text-decoration: none;
  font-weight: 600;
}

.breadcrumb .sep { opacity: 0.5; }

.chapter-nav-top {
  margin-left: auto;
  display: flex;
  gap: 6px;
  font-size: 12px;
}

.chapter-nav-top a {
  background: rgba(22, 163, 74, 0.1);
  padding: 4px 10px;
  border-radius: 999px;
  color: var(--green) !important;
  font-weight: 700;
}

/* ============================================================
   8. HERO
   ============================================================ */
.hero {
  position: relative;
  overflow: hidden;
  padding: 60px 24px 68px;
  background:
    radial-gradient(900px 500px at 10% -10%, rgba(5, 150, 105, 0.16), transparent 62%),
    radial-gradient(800px 480px at 95% 10%, rgba(8, 145, 178, 0.14), transparent 60%),
    linear-gradient(180deg, #ffffff 0%, var(--bg-alt) 100%);
  border-bottom: 1px solid var(--border);
  text-align: center;
}

.hero-inner {
  position: relative;
  z-index: 1;
  max-width: 900px;
  margin: 0 auto;
}

.hero > div { position: relative; z-index: 1; }

.hero-particles {
  position: absolute;
  inset: 0;
  pointer-events: none;
  z-index: 0;
}

.eyebrow {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 700;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--green);
  background: rgba(5, 150, 105, 0.08);
  border: 1px solid rgba(5, 150, 105, 0.18);
  padding: 7px 16px;
  border-radius: 999px;
  margin-bottom: 22px;
}

.legacy-badge {
  display: inline-block;
  font: 700 11px var(--ui);
  background: #fef3c7;
  color: #92400e;
  border: 1px solid #fbbf24;
  padding: 4px 12px;
  border-radius: 999px;
  margin-left: 8px;
  vertical-align: middle;
}

html.dark .legacy-badge {
  background: rgba(217, 119, 6, 0.3);
  color: #fcd34d;
  border-color: rgba(217, 119, 6, 0.5);
}

.eyebrow .dot {
  width: 7px;
  height: 7px;
  border-radius: 50%;
  background: var(--green);
  box-shadow: 0 0 0 3px rgba(5, 150, 105, 0.18);
  animation: pulse 2s ease-in-out infinite;
}

@keyframes pulse {
  0%, 100% { opacity: 1; transform: scale(1); }
  50% { opacity: 0.6; transform: scale(1.15); }
}

.hero h1 {
  font-family: var(--serif);
  font-size: clamp(1.8rem, 5vw, 3.1rem);
  line-height: 1.1;
  font-weight: 700;
  letter-spacing: -0.02em;
  margin-bottom: 16px;
}

.hero h1 .accent,
.hero h1 span {
  background: linear-gradient(120deg, var(--green), var(--cyan), var(--purple));
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}

.hero .lede,
.lede {
  font-size: 1.06rem;
  color: var(--muted);
  max-width: 680px;
  margin: 0 auto 26px;
  line-height: 1.75;
}

.hero-meta,
.meta-info {
  display: flex;
  gap: 12px;
  flex-wrap: wrap;
  justify-content: center;
  font-family: var(--ui);
  font-size: 12.5px;
  font-weight: 600;
  color: var(--muted);
  margin-top: 8px;
}

.meta-chip {
  font-family: var(--ui);
  font-size: 12.5px;
  font-weight: 600;
  background: rgba(255, 255, 255, 0.85);
  border: 1px solid var(--border);
  padding: 8px 16px;
  border-radius: 999px;
  color: var(--ink);
  display: inline-flex;
  align-items: center;
  gap: 8px;
}

.meta-chip .mini-dot {
  width: 6px;
  height: 6px;
  border-radius: 50%;
  background: var(--green);
}

.meta-info span {
  display: flex;
  align-items: center;
  gap: 5px;
}

/* ============================================================
   9. STATS BAND
   ============================================================ */
.stats-band,
.stats {
  max-width: 1040px;
  margin: -30px auto 0;
  padding: 0 24px;
  position: relative;
  z-index: 3;
}

.stats-inner,
.si {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 20px;
  padding: 22px 28px;
  box-shadow: var(--shadow-lg);
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(130px, 1fr));
  gap: 16px;
  text-align: center;
}

.stat-item,
.si > div {
  text-align: center;
  padding: 4px 10px;
  border-right: 1px solid var(--border);
}

.stat-item:last-child,
.si > div:last-child { border-right: 0; }

.stat-num,
.sn {
  font-family: var(--serif);
  font-size: clamp(1.5rem, 3.2vw, 2rem);
  font-weight: 800;
  line-height: 1;
  background: linear-gradient(135deg, var(--green), var(--cyan));
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
  display: block;
  margin-bottom: 6px;
}

.stat-label,
.sl {
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 600;
  color: var(--muted);
  line-height: 1.4;
}

/* ============================================================
   10. CATEGORY NAV
   ============================================================ */
.cat-nav {
  position: relative;
  z-index: 800;
  background: rgba(248, 249, 252, 0.94);
  border-bottom: 1px solid var(--border);
  padding: 12px 24px;
  margin-top: 30px;
}

.cat-nav-inner {
  max-width: 1080px;
  margin: 0 auto;
  display: flex;
  gap: 6px;
  justify-content: center;
  flex-wrap: wrap;
}

.cat-tab {
  font-family: var(--ui);
  font-size: 13px;
  font-weight: 700;
  padding: 9px 18px;
  border-radius: 999px;
  border: 1.5px solid var(--border);
  background: #fff;
  color: var(--muted);
  text-decoration: none;
  transition: all 0.22s ease;
  display: inline-flex;
  align-items: center;
  gap: 7px;
}

.cat-tab:hover {
  border-color: var(--green);
  color: var(--green);
}

.cat-tab.active {
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  border-color: transparent;
  box-shadow: 0 6px 16px rgba(5, 150, 105, 0.3);
}

/* ============================================================
   11. SECTION LAYOUT
   ============================================================ */
.section { padding: 68px 24px; }

.wrap,
.w,
.wrap-narrow {
  max-width: 1140px;
  margin: 0 auto;
}

.wrap-narrow { max-width: 900px; }

.alt {
  background: var(--bg-alt);
  border-block: 1px solid var(--border);
}

.topic { padding: 68px 0; }

.sh { margin-bottom: 34px; }

.sh.center,
.sh.c { text-align: center; }

.sh h2 {
  font-family: var(--serif);
  font-size: clamp(1.6rem, 3.6vw, 2.2rem);
  font-weight: 700;
  letter-spacing: -0.015em;
  line-height: 1.2;
  margin-bottom: 8px;
}

.sh p { color: var(--muted); font-size: 1rem; }

.kicker {
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 700;
  letter-spacing: 0.18em;
  text-transform: uppercase;
  color: var(--green);
  margin-bottom: 8px;
  display: block;
}

.sec-head { margin-bottom: 38px; }

.sec-head.center { text-align: center; }

.sec-head h2 {
  font-family: var(--serif);
  font-size: clamp(1.6rem, 3.6vw, 2.2rem);
  font-weight: 700;
  letter-spacing: -0.015em;
  line-height: 1.2;
  margin-bottom: 10px;
}

.sec-head p { color: var(--muted); font-size: 1rem; }

.sec-actions {
  display: flex;
  justify-content: flex-end;
  margin-bottom: 16px;
}

.read-btn {
  font: 600 12px var(--ui);
  padding: 6px 14px;
  border-radius: 999px;
  border: 1.5px solid var(--border);
  background: var(--card);
  color: var(--muted);
  cursor: pointer;
  transition: 0.2s;
  display: inline-flex;
  align-items: center;
  gap: 6px;
}

.read-btn:hover {
  border-color: var(--green);
  color: var(--green);
}

.read-btn.done {
  background: rgba(5, 150, 105, 0.1);
  border-color: var(--green);
  color: var(--green);
}

/* ============================================================
   12. CONTENT CARDS
   ============================================================ */
.content-card,
.card,
.cc,
.topic-head {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 18px;
  box-shadow: var(--shadow-sm);
  transition: all 0.25s ease;
}

.content-card,
.card,
.cc { padding: 28px 30px; margin-bottom: 20px; }

.content-card:hover,
.card:hover,
.cc:hover { box-shadow: var(--shadow-md); }

.card h3,
.cc h3,
.content-card h3 {
  font-family: var(--serif);
  font-size: 1.28rem;
  font-weight: 700;
  margin-bottom: 14px;
  color: var(--ink);
  display: flex;
  align-items: center;
  gap: 12px;
  flex-wrap: wrap;
}

.card h4,
.cc h4,
.content-card h4 {
  font-family: var(--ui);
  font-size: 15px;
  font-weight: 700;
  margin: 18px 0 8px;
  color: var(--green);
}

.card p,
.cc p,
.content-card p,
.card li,
.cc li,
.content-card li {
  color: #334155;
  margin-bottom: 12px;
  line-height: 1.8;
}

html.dark .card p,
html.dark .cc p,
html.dark .card li,
html.dark .cc li {  color: #cbd5e1; }

.card ul,
.cc ul,
.content-card ul,
.card ol,
.cc ol,
.content-card ol {
  padding-left: 24px;
  margin-bottom: 12px;
}

.card li strong,
.cc li strong,
.content-card li strong { color: var(--ink); }

.badge {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: #fff;
  padding: 4px 11px;
  border-radius: 999px;
  background: linear-gradient(135deg, var(--green), var(--cyan));
  letter-spacing: 0.05em;
  box-shadow: 0 2px 8px rgba(5, 150, 105, 0.3);
}

/* Topic head with number */
.topic-head {
  display: flex;
  align-items: flex-start;
  gap: 14px;
  margin-bottom: 28px;
  padding: 20px 24px;
}

.num {
  width: 54px;
  height: 54px;
  border-radius: 14px;
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  display: grid;
  place-items: center;
  font-family: var(--ui);
  font-weight: 800;
  flex: none;
  box-shadow: 0 8px 18px rgba(5, 150, 105, 0.22);
}

.topic-head h2 {
  font-family: var(--serif);
  font-size: clamp(1.6rem, 3.6vw, 2.2rem);
  font-weight: 700;
  line-height: 1.2;
  margin-bottom: 8px;
}

.topic-head p { color: var(--muted); }

/* ===== NEW (FIX): Sub-topic card (nested under .card) — used in Ch 2 ===== */
.subtopic {
  margin-left: 8px;
  border-left: 3px solid rgba(5, 150, 105, 0.22);
  padding-left: 18px;
}

html.dark .subtopic {
  border-left-color: rgba(34, 197, 94, 0.32);
}

@media (max-width: 720px) {
  .subtopic {
    margin-left: 0;
    padding-left: 14px;
  }
}

/* ===== Coverage strip (Chapter 1) ===== */
.coverage-strip {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin: 0 0 22px;
  padding: 14px 18px;
  background: var(--bg-alt);
  border-radius: 14px;
  border: 1px solid var(--border);
}

.coverage-strip span {
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 700;
  padding: 6px 14px;
  background: var(--card);
  border-radius: 999px;
  color: var(--green);
  border: 1px solid rgba(5, 150, 105, 0.2);
  box-shadow: var(--shadow-sm);
}

html.dark .coverage-strip { background: #0f2318; }
html.dark .coverage-strip span {
  background: #14291c;
  color: #86efac;
  border-color: rgba(34, 197, 94, 0.3);
}

/* ===== Topic-visual & integrated-viz-card modifiers ===== */
.topic-visual { margin-top: 16px; }
.integrated-viz-card { margin-top: 16px; }

/* ============================================================
   13. INFO BOXES
   ============================================================ */
.def,
.note,
.exam,
.real,
.mistake,
.why,
.analogy,
.trap,
.try,
.takeaway,
.did-you-know,
.mnemonic-box,
.def-box,
.exam-alert,
.topic-note {
  border-radius: 0 14px 14px 0;
  padding: 16px 22px;
  margin: 16px 0;
  position: relative;
}

.def,
.def-box {
  background: linear-gradient(135deg, rgba(5, 150, 105, 0.05), rgba(8, 145, 178, 0.04));
  border-left: 4px solid var(--green);
}

.def i,
.def-label,
.def-box .def-label {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: var(--green);
  letter-spacing: 0.12em;
  text-transform: uppercase;
  display: block;
  margin-bottom: 8px;
  font-style: normal;
}

.def p,
.def-box p { margin: 0 0 6px; color: var(--ink); }

/* ===== NEW: Callout boxes (Chapter 2) ===== */
.callout {
  border-radius: 14px;
  padding: 16px 22px;
  margin: 16px 0;
  border-left: 4px solid;
  font-size: 15.5px;
  line-height: 1.7;
  background: var(--card);
  color: var(--ink);
}

.callout strong {
  font-weight: 800;
  color: var(--green);
  margin-right: 4px;
}

.callout.definition,
.callout.def {
  background: linear-gradient(135deg, rgba(5, 150, 105, 0.05), rgba(8, 145, 178, 0.04));
  border-left-color: var(--green);
}

.callout.definition strong,
.callout.def strong { color: var(--green); }

.callout.exam,
.callout.exam-alert {
  background: linear-gradient(135deg, #fff7ed, #fffbeb);
  border-left-color: var(--gold);
  color: #334155;
}

.callout.exam strong { color: #b45309; }

.callout.warning {
  background: linear-gradient(135deg, #fef9c3, #fef3c7);
  border-left-color: var(--gold);
  color: #78350f;
}

.callout.warning strong { color: #b45309; }

.callout.tip {
  background: linear-gradient(135deg, #dcfce7, #d1fae5);
  border-left-color: var(--green);
  color: #14532d;
}

.callout.tip strong { color: #166534; }

html.dark .callout.definition,
html.dark .callout.def {
  background: linear-gradient(135deg, rgba(22, 163, 74, 0.12), rgba(13, 148, 136, 0.08));
  color: var(--ink);
}
html.dark .callout.definition strong,
html.dark .callout.def strong { color: #86efac; }

html.dark .callout.exam,
html.dark .callout.exam-alert {
  background: linear-gradient(135deg, #0f2318, #14291c);
  color: #86efac;
}
html.dark .callout.exam strong { color: #fbbf24; }

html.dark .callout.warning {
  background: linear-gradient(135deg, #2a2308, #3a2e0a);
  color: #fcd34d;
}
html.dark .callout.warning strong { color: #fbbf24; }

html.dark .callout.tip {
  background: linear-gradient(135deg, #0f2318, #14291c);
  color: #86efac;
}
html.dark .callout.tip strong { color: #4ade80; }

.note {
  background: rgba(8, 145, 178, 0.06);
  border: 1px solid rgba(8, 145, 178, 0.18);
  border-left: 4px solid var(--cyan);
  border-radius: 14px;
}

.note i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: var(--cyan);
  letter-spacing: 0.12em;
  text-transform: uppercase;
  display: block;
  margin-bottom: 6px;
  font-style: normal;
}

.exam,
.exam-alert {
  background: linear-gradient(135deg, #fff7ed, #fffbeb);
  border-left: 4px solid var(--gold);
  color: #334155;
}

.real {
  background: linear-gradient(135deg, #fef9c3, #fef3c7);
  border-left: 4px solid var(--gold);
}

.real i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: #b45309;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 10px;
  font-style: normal;
}

.real i::before {
  content: "🌍";
  font-size: 14px;
}

.real p {
  margin: 0 0 8px;
  color: #78350f;
  font-size: 15.5px;
}

.real ul {
  margin: 6px 0 0;
  padding-left: 22px;
  color: #78350f;
  font-size: 15.5px;
}

.real ul li { margin-bottom: 6px; }

.mistake {
  background: linear-gradient(135deg, #fee2e2, #fecaca);
  border-left: 4px solid var(--red);
}

.mistake i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: #991b1b;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 10px;
  font-style: normal;
}

.mistake i::before {
  content: "⚠️";
  font-size: 14px;
}

.mistake p {
  color: #7f1d1d;
  font-size: 15.5px;
  margin: 0 0 6px;
}

.why {
  background: linear-gradient(135deg, #dcfce7, #d1fae5);
  border-left: 4px solid var(--green);
}

.why i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: #166534;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 10px;
  font-style: normal;
}

.why i::before {
  content: "💡";
  font-size: 14px;
}

.why p {
  color: #14532d;
  font-size: 15.5px;
  margin: 0;
}

.analogy {
  background: linear-gradient(135deg, #fce7f3, #fbcfe8);
  border-left: 4px solid #ec4899;
}

.analogy i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: #9d174d;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 10px;
  font-style: normal;
}

.analogy i::before {
  content: "🎯";
  font-size: 14px;
}

.analogy p {
  color: #831843;
  font-size: 15.5px;
  margin: 0;
}

.trap {
  background: linear-gradient(135deg, #ffedd5, #fed7aa);
  border-left: 4px solid #ea580c;
}

.trap i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: #9a3412;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 10px;
  font-style: normal;
}

.trap i::before {
  content: "🎯";
  font-size: 14px;
}

.trap p {
  color: #7c2d12;
  font-size: 15.5px;
  margin: 0;
}

.try {
  background: linear-gradient(135deg, #d1fae5, #a7f3d0);
  border-left: 4px solid var(--green);
}

.try i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: #065f46;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 10px;
  font-style: normal;
}

.try i::before {
  content: "🧪";
  font-size: 14px;
}

.try p,
.try ul {
  color: #064e3b;
  font-size: 15.5px;
  margin: 0 0 6px;
}

.try ul { padding-left: 22px; }

.takeaway {
  background: linear-gradient(135deg, #dcfce7, #d1fae5);
  border-left: 4px solid var(--green);
  margin-top: 24px;
  padding: 14px 20px;
}

.takeaway i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: #166534;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 8px;
  font-style: normal;
}

.takeaway i::before {
  content: "🔑";
  font-size: 14px;
}

.takeaway p {
  margin: 0;
  color: #14532d;
  font-size: 15.5px;
  font-weight: 500;
}

.did-you-know {
  background: linear-gradient(135deg, #dbeafe, #e0f2fe);
  border: 2px dashed var(--cyan);
  border-radius: 14px;
  padding: 16px 22px;
  margin: 20px 0;
}

.did-you-know i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: var(--cyan);
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 8px;
  font-style: normal;
}

.did-you-know i::before {
  content: "💎";
  font-size: 14px;
}

.did-you-know p {
  margin: 0;
  color: #0c4a6e;
  font-size: 15px;
  line-height: 1.65;
}

.mnemonic-box {
  background: linear-gradient(135deg, #fef3c7, #fde68a);
  border: 2px solid #f59e0b;
  border-left: 4px solid var(--gold);
  border-radius: 0 14px 14px 0;
  padding: 16px 22px;
  margin: 16px 0;
}

.mnemonic-box i {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: #92400e;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 10px;
  font-style: normal;
}

.mnemonic-box i::before {
  content: "🧠";
  font-size: 14px;
}

.mnemonic-box p,
.mnemonic-box ul {
  margin: 0 0 8px;
  color: #78350f;
  font-size: 15.5px;
  line-height: 1.7;
}

.mnemonic-box ul { padding-left: 22px; }

.mnemonic-box strong { color: #7c2d12; }

.topic-note {
  background: linear-gradient(135deg, rgba(14, 116, 144, 0.06), rgba(5, 150, 105, 0.05));
  border: 1px solid rgba(8, 145, 178, 0.18);
  border-radius: 14px;
  padding: 16px 18px;
  margin: 16px 0;
}

.topic-note-label {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: var(--cyan);
  letter-spacing: 0.12em;
  text-transform: uppercase;
  display: block;
  margin-bottom: 7px;
}

/* Formula box */
.formula-box {
  background: linear-gradient(135deg, #ecfdf5, #f0f9ff);
  border: 1.5px solid rgba(5, 150, 105, 0.15);
  border-radius: 14px;
  padding: 18px 22px;
  margin: 14px 0;
  text-align: center;
  font-size: 1.02rem;
  overflow-x: auto;
}

/* ============================================================
   14. TABLES
   ============================================================ */
.table-wrap,
.tw {
  overflow-x: auto;
  border-radius: 14px;
  border: 1px solid var(--border);
  box-shadow: var(--shadow-sm);
  background: var(--card);
  margin: 18px 0;
}

table {
  width: 100%;
  border-collapse: collapse;
  min-width: 560px;
  font-size: 15px;
}

thead {
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  font-family: var(--ui);
}

thead th,
th {
  padding: 12px 16px;
  text-align: left;
  font-weight: 700;
  font-size: 12.5px;
  letter-spacing: 0.05em;
  text-transform: uppercase;
}

tbody tr {
  border-bottom: 1px solid var(--border);
  transition: background 0.2s ease;
}

tbody tr:last-child { border-bottom: 0; }
tbody tr:hover { background: var(--bg-alt); }

td {
  padding: 12px 16px;
  color: #334155;
  border-bottom: 1px solid var(--border);
  vertical-align: top;
  line-height: 1.7;
}

html.dark td { color: #cbd5e1; }

td:first-child {
  font-weight: 700;
  color: var(--ink);
}

/* ===== Dihybrid Punnett table (Chapter 4) ===== */
.pun {
  width: 100%;
  max-width: 620px;
  margin: 0 auto;
  border-collapse: collapse;
  min-width: 520px;
  font-family: var(--ui);
  font-size: 13px;
}

.pun th,
.pun td {
  padding: 8px 10px;
  text-align: center;
  border: 1px solid var(--border);
  font-weight: 700;
  color: var(--ink);
}

.pun th {
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  font-size: 12.5px;
  letter-spacing: 0.04em;
}

.pun .c1 { background: #fef9c3; }
.pun .c2 { background: #fed7aa; }
.pun .c3 { background: #bbf7d0; }
.pun .c4 { background: #e2e8f0; }

html.dark .pun td { color: #cbd5e1; }
html.dark .pun .c1 { background: rgba(254, 249, 195, 0.15); }
html.dark .pun .c2 { background: rgba(253, 186, 116, 0.15); }
html.dark .pun .c3 { background: rgba(187, 247, 208, 0.15); }
html.dark .pun .c4 { background: rgba(226, 232, 240, 0.15); }

/* ===== Genetic code codon table (Chapter 5) ===== */
.gc {
  width: 100%;
  border-collapse: collapse;
  min-width: 640px;
  font-family: var(--ui);
  font-size: 12px;
  background: var(--card);
  border-radius: 12px;
  overflow: hidden;
  box-shadow: var(--shadow-sm);
}

.gc th,
.gc td {
  padding: 6px 8px;
  border: 1px solid var(--border);
  vertical-align: top;
  color: var(--ink);
  line-height: 1.55;
}

.gc th {
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  font-weight: 700;
  font-size: 12px;
  letter-spacing: 0.04em;
  text-align: center;
}

.gc td:first-child {
  background: var(--bg-alt);
  font-weight: 800;
  color: var(--green);
  text-align: center;
  width: 60px;
}

.gc span.st { color: #be123c; font-weight: 800; }
.gc span.in { color: #059669; font-weight: 800; }

html.dark .gc { background: #14291c; }
html.dark .gc td { color: #cbd5e1; }
html.dark .gc td:first-child { background: #0f2318; color: #86efac; }
html.dark .gc span.st { color: #f87171; }
html.dark .gc span.in { color: #4ade80; }

/* ============================================================
   15. VISUALIZATION BOXES (SVG)
   ============================================================ */
.viz-box,
.viz {
  background: linear-gradient(135deg, #fafbfe, #f4f6fb);
  border: 1.5px solid rgba(5, 150, 105, 0.15);
  border-radius: 16px;
  padding: 22px 18px;
  margin: 20px 0;
  text-align: center;
  position: relative;
  overflow-x: auto;
  overflow-y: hidden;
  -webkit-overflow-scrolling: touch;
}

.viz-box .viz-label,
.viz > b {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: var(--green);
  letter-spacing: 0.14em;
  text-transform: uppercase;
  display: block;
  margin-bottom: 12px;
}

.viz-box svg,
.viz svg {
  max-width: 100%;
  min-width: 640px;
  height: auto;
  display: block;
  margin: 0 auto;
  border-radius: 10px;
}

.viz-caption,
.cap {
  font-family: var(--ui);
  font-size: 13px;
  color: var(--muted);
  margin-top: 14px;
  line-height: 1.6;
  font-style: italic;
}

/* Bio SVG safety */
.bio-svg {
  width: 100%;
  max-width: 1000px;
  height: auto;
  display: block;
  margin: 0 auto;
  overflow: visible;
}

.bio-svg text {
  font-family: Inter, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  paint-order: stroke fill;
  stroke: rgba(255, 255, 255, 0.62);
  stroke-width: 0.65px;
  stroke-linejoin: round;
}

.bio-svg .title {
  font-family: "Playfair Display", Georgia, serif;
  font-weight: 800;
  font-size: 15px;
  fill: #0f172a;
  stroke: none;
}

.bio-svg .label {
  font-weight: 800;
  font-size: 11.5px;
  fill: #0f172a;
}

.bio-svg .small {
  font-size: 9.5px;
  fill: #64748b;
  font-weight: 600;
}

.bio-svg.expert-svg {
  display: block;
  width: 100%;
  height: auto;
  min-height: 220px;
  background: #fdfbf6;
  border: 1px solid rgba(15, 23, 42, 0.08);
  border-radius: 16px;
  overflow: visible;
}

.realistic-svg {
  shape-rendering: geometricPrecision;
  text-rendering: geometricPrecision;
}

/* ============================================================
   16. INTERACTIVE / CALCULATORS
   ============================================================ */
.interactive {
  background: var(--card);
  border: 1.5px solid rgba(5, 150, 105, 0.3);
  border-radius: 16px;
  padding: 24px;
  margin: 20px 0;
  box-shadow: var(--shadow-sm);
}

.interactive h4 {
  font-family: var(--ui);
  font-size: 15px;
  font-weight: 800;
  color: var(--green);
  margin-bottom: 16px;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  display: flex;
  align-items: center;
  gap: 8px;
}

.interactive h4::before {
  content: "🎮";
  font-size: 18px;
}

.calc-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 14px;
  margin-bottom: 16px;
}

.calc-field {
  display: flex;
  flex-direction: column;
  gap: 4px;
}

.calc-field label {
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 600;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.08em;
}

.calc-input,
select {
  width: 100%;
  padding: 10px 14px;
  font-family: var(--ui);
  font-size: 14px;
  font-weight: 600;
  border: 1.5px solid var(--border);
  border-radius: 10px;
  background: #fff;
  color: var(--ink);
  transition: border-color 0.2s;
}

.calc-input:focus,
select:focus {
  outline: none;
  border-color: var(--green);
  box-shadow: 0 0 0 3px rgba(5, 150, 105, 0.15);
}

.calc-btn {
  font-family: var(--ui);
  font-size: 14px;
  font-weight: 700;
  padding: 12px 24px;
  border-radius: 10px;
  border: none;
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  cursor: pointer;
  transition: 0.2s;
  box-shadow: 0 4px 14px rgba(5, 150, 105, 0.3);
  width: 100%;
}

.calc-btn:hover {
  transform: translateY(-1px);
  box-shadow: 0 6px 20px rgba(5, 150, 105, 0.4);
}

.calc-results {
  background: linear-gradient(135deg, #dcfce7, #d1fae5);
  padding: 16px;
  border-radius: 12px;
  margin-top: 16px;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 10px;
}

.calc-results .res-item {
  background: #fff;
  padding: 10px 14px;
  border-radius: 10px;
  border: 1px solid rgba(5, 150, 105, 0.15);
}

.calc-results .lbl {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 700;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.08em;
}

.calc-results .val {
  font-size: 15px;
  font-weight: 800;
  color: var(--green);
  display: block;
  margin-top: 2px;
  font-family: var(--body);
}

/* ============================================================
   17. SHOW ANSWER ACCORDIONS
   ============================================================ */
.show-answer-wrap,
.sa {
  margin: 16px 0;
  border-radius: 14px;
  overflow: hidden;
  border: 1.5px solid rgba(5, 150, 105, 0.2);
  background: var(--card);
}

.show-answer-btn,
.sa button {
  font-family: var(--ui);
  font-size: 14px;
  font-weight: 700;
  width: 100%;
  text-align: left;
  background: linear-gradient(135deg, rgba(5, 150, 105, 0.08), rgba(8, 145, 178, 0.06));
  color: var(--green);
  border: none;
  padding: 14px 20px;
  cursor: pointer;
  display: flex;
  align-items: center;
  gap: 10px;
  transition: all 0.22s ease;
  letter-spacing: 0.02em;
}

.show-answer-btn:hover,
.sa button:hover {
  background: linear-gradient(135deg, rgba(5, 150, 105, 0.14), rgba(8, 145, 178, 0.12));
}

.show-answer-btn .arrow,
.sa button .arrow,
.show-answer-btn .icon,
.sa button span {
  display: inline-block;
  transition: transform 0.28s ease;
  font-size: 12px;
  color: var(--green);
  width: 14px;
  flex: none;
}

.show-answer-wrap.open .show-answer-btn .arrow,
.sa.open button span {
  transform: rotate(90deg);
}

.show-answer-body,
.sa .b {
  display: none;
  padding: 22px 24px;
  border-top: 1px dashed rgba(5, 150, 105, 0.25);
  background: #fefeff;
  animation: slideDown 0.3s ease;
}

.show-answer-wrap.open .show-answer-body,
.sa.open .b {
  display: block;
}

@keyframes slideDown {
  from { opacity: 0; transform: translateY(-8px); }
  to { opacity: 1; transform: translateY(0); }
}

.step-block,
.step {
  background: #f8fafc;
  border-left: 4px solid var(--green);
  border-radius: 0 12px 12px 0;
  padding: 14px 18px;
  margin: 12px 0;
}

.step-block .step-header,
.step h6 {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  margin-bottom: 8px;
  font-family: var(--ui);
}

.step-block .step-name,
.step h6 {
  font-size: 13px;
  font-weight: 800;
  color: var(--green);
  letter-spacing: 0.06em;
  text-transform: uppercase;
}

.step-block .step-mark,
.step h6 em {
  font-style: normal;
  font-size: 11.5px;
  font-weight: 700;
  background: linear-gradient(135deg, var(--green), #10b981);
  color: #fff;
  padding: 3px 10px;
  border-radius: 999px;
  letter-spacing: 0.04em;
  white-space: nowrap;
}

.step-block p,
.step p {
  margin: 6px 0;
  color: #334155;
  font-size: 15px;
  line-height: 1.7;
  overflow-x: auto;
}

html.dark .step p { color: #cbd5e1; }

.final-answer,
.fa {
  background: linear-gradient(135deg, rgba(5, 150, 105, 0.1), rgba(16, 185, 129, 0.08));
  border: 1.5px solid rgba(5, 150, 105, 0.3);
  border-radius: 12px;
  padding: 12px 18px;
  margin-top: 14px;
  font-weight: 600;
  color: #065f46;
  display: flex;
  align-items: center;
  gap: 10px;
}

.fa::before {
  content: "✓";
  width: 24px;
  height: 24px;
  display: grid;
  place-items: center;
  background: var(--green);
  color: #fff;
  border-radius: 50%;
  font-weight: 800;
  flex: none;
  font-size: 13px;
}

.marks-total {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 700;
  background: rgba(5, 150, 105, 0.1);
  color: var(--green);
  padding: 5px 12px;
  border-radius: 999px;
  margin-bottom: 12px;
}

.mt {
  display: inline-block;
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 700;
  background: rgba(5, 150, 105, 0.1);
  color: var(--green);
  padding: 5px 12px;
  border-radius: 999px;
  margin-bottom: 12px;
}

/* ============================================================
   18. MCQ / QUIZ
   ============================================================ */
.mcq-form,
.quiz,
.qf {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 18px;
  padding: 28px;
  box-shadow: var(--shadow-sm);
}

.q {
  padding: 20px 0;
  border-bottom: 1px dashed var(--border);
}

.q:first-child { padding-top: 0; }

.q:last-of-type {
  border-bottom: 0;
  padding-bottom: 12px;
}

.q-text,
.qt {
  font-size: 16px;
  font-weight: 600;
  margin-bottom: 14px;
  line-height: 1.6;
  display: flex;
  gap: 10px;
  color: var(--ink);
}

.q-text span,
.qt span {
  color: var(--green);
  font-family: var(--ui);
  font-weight: 800;
  flex: none;
}

.opts,
.o {
  display: grid;
  gap: 8px;
  padding-left: 22px;
}

.o {
  padding-left: 0;
  margin-top: 10px;
}

.opts label,
.o label {
  display: flex;
  align-items: flex-start;
  gap: 10px;
  padding: 10px 14px;
  border-radius: 10px;
  border: 1.5px solid var(--border);
  background: #fefefe;
  cursor: pointer;
  font-size: 15px;
  line-height: 1.5;
  transition: all 0.18s ease;
  color: var(--ink);
}

html.dark .opts label,
html.dark .o label {
  background: #0f2318;
  color: #cbd5e1;
}

.opts label:hover,
.o label:hover {
  border-color: var(--green);
  background: rgba(5, 150, 105, 0.03);
}

.opts input,
.o input {
  accent-color: var(--green);
  cursor: pointer;
  flex: none;
  margin-top: 3px;
}

.opts span {
  font-family: var(--ui);
  font-weight: 700;
  color: var(--muted);
  font-size: 13.5px;
  flex: none;
}

.opts label.correct,
.o label.correct,
.o .correct {
  border-color: var(--green);
  background: rgba(5, 150, 105, 0.08);
}

.opts label.correct span { color: var(--green); }

.opts label.wrong,
.o label.wrong,
.o .wrong {
  border-color: var(--red);
  background: rgba(220, 38, 38, 0.07);
}

.opts label.wrong span { color: var(--red); }

.explain,
.ex {
  display: none;
  margin: 14px 0 0 22px;
  padding: 12px 18px;
  background: rgba(5, 150, 105, 0.07);
  border-left: 3px solid var(--green);
  border-radius: 0 10px 10px 0;
  font-size: 14.5px;
  color: #334155;
  line-height: 1.7;
}

.ex { margin-left: 0; }

.explain.show,
.ex.show { display: block; }

.mcq-actions,
.act {
  display: flex;
  gap: 12px;
  align-items: center;
  flex-wrap: wrap;
  margin-top: 26px;
  padding-top: 24px;
  border-top: 1px solid var(--border);
}

.btn {
  font-family: var(--ui);
  font-size: 14px;
  font-weight: 600;
  padding: 12px 26px;
  border-radius: 999px;
  border: 1.5px solid transparent;
  cursor: pointer;
  display: inline-flex;
  align-items: center;
  gap: 8px;
  transition: all 0.22s ease;
  background: var(--card);
  color: var(--ink);
}

.btn-primary,
.btn.p {
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  border-color: transparent;
  box-shadow: 0 6px 18px rgba(5, 150, 105, 0.28);
}

.btn-primary:hover,
.btn.p:hover {
  transform: translateY(-2px);
  box-shadow: 0 10px 26px rgba(5, 150, 105, 0.38);
  color: #fff;
}

.btn-outline,
.btn:not(.p):not(.btn-primary) {
  background: var(--card);
  color: var(--ink);
  border-color: var(--border);
}

.btn-outline:hover,
.btn:not(.p):not(.btn-primary):hover {
  border-color: var(--green);
  color: var(--green);
}

.score {
  margin-left: auto;
  font-family: var(--ui);
  font-size: 14.5px;
  font-weight: 700;
  color: var(--muted);
  background: var(--bg-alt);
  padding: 10px 20px;
  border-radius: 10px;
  border: 1px solid var(--border);
}

.score b {
  color: var(--green);
  font-size: 18px;
}

.result-banner,
.rb {
  display: none;
  margin-top: 20px;
  padding: 20px 24px;
  border-radius: 12px;
  font-family: var(--ui);
  font-size: 15px;
  line-height: 1.6;
  text-align: center;
}

.result-banner.show,
.rb.show {
  display: block;
  animation: fadeIn 0.3s ease;
}

.result-banner.good,
.rb.show {
  background: rgba(5, 150, 105, 0.1);
  border: 1px solid rgba(5, 150, 105, 0.3);
  color: #065f46;
}

.result-banner.mid {
  background: rgba(217, 119, 6, 0.12);
  border: 1px solid rgba(217, 119, 6, 0.3);
  color: #78350f;
}

.result-banner.low {
  background: rgba(220, 38, 38, 0.08);
  border: 1px solid rgba(220, 38, 38, 0.25);
  color: #991b1b;
}

.result-banner .rt {
  font-size: 1.15rem;
  font-weight: 800;
  display: block;
  margin-bottom: 4px;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(-6px); }
  to { opacity: 1; transform: none; }
}

/* Quiz answer display (Chapter 2) */
.ans {
  display: none;
  margin: 14px 0 0;
  padding: 12px 18px;
  background: rgba(5, 150, 105, 0.07);
  border-left: 3px solid var(--green);
  border-radius: 0 10px 10px 0;
  font-size: 14.5px;
  color: #334155;
  line-height: 1.7;
  font-family: var(--body);
}

html.dark .ans {
  background: rgba(34, 197, 94, 0.12);
  color: #cbd5e1;
}

.btn.check {
  padding: 12px 26px;
  border-radius: 999px;
  font-family: var(--ui);
  font-size: 14px;
  font-weight: 600;
  width: auto;
  display: inline-flex;
  align-items: center;
  gap: 8px;
  margin-top: 10px;
}

/* ============================================================
   19. ASSERTION-REASON CARDS
   ============================================================ */
.ar-card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 16px;
  padding: 22px 24px;
  margin-bottom: 16px;
  box-shadow: var(--shadow-sm);
  transition: all 0.25s ease;
}

.ar-card:hover { box-shadow: var(--shadow-md); }

.ar-qnum {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 800;
  width: 30px;
  height: 30px;
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  border-radius: 10px;
  margin-bottom: 12px;
}

.ar-assertion,
.ar-reason,
.ar {
  padding: 12px 16px;
  border-radius: 10px;
  margin: 8px 0;
  font-size: 15.5px;
  line-height: 1.7;
  color: var(--ink);
}

.ar-assertion,
.ar {
  background: rgba(5, 150, 105, 0.06);
  border-left: 3px solid var(--green);
}

.ar-reason,
.ar.r {
  background: rgba(217, 119, 6, 0.06);
  border-left: 3px solid var(--gold);
}

.ar-assertion strong,
.ar-reason strong,
.ar b {
  font-family: var(--ui);
  font-size: 11.5px;
  font-weight: 800;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  display: block;
  margin-bottom: 4px;
}

.ar-assertion strong { color: var(--green); }
.ar-reason strong { color: var(--gold); }

.ar-options {
  display: grid;
  gap: 8px;
  margin-top: 14px;
  padding-left: 0;
}

.ar-options label {
  display: flex;
  align-items: flex-start;
  gap: 10px;
  padding: 10px 14px;
  border-radius: 10px;
  border: 1.5px solid var(--border);
  background: #fefefe;
  cursor: pointer;
  font-size: 14.5px;
  line-height: 1.55;
  transition: all 0.18s ease;
  color: var(--ink);
}

html.dark .ar-options label {
  background: #0f2318;
  color: #cbd5e1;
}

.ar-options label:hover {
  border-color: var(--green);
  background: rgba(5, 150, 105, 0.03);
}

.ar-options input {
  accent-color: var(--green);
  cursor: pointer;
  margin-top: 3px;
  flex: none;
}

.ar-options label.correct {
  border-color: var(--green);
  background: rgba(5, 150, 105, 0.08);
}

.ar-options label.wrong {
  border-color: var(--red);
  background: rgba(220, 38, 38, 0.07);
}

.ar-explain {
  display: none;
  margin-top: 14px;
  padding: 14px 18px;
  background: rgba(5, 150, 105, 0.06);
  border-left: 3px solid var(--green);
  border-radius: 0 10px 10px 0;
  font-size: 14.5px;
  line-height: 1.7;
  color: #334155;
}

.ar-explain.show { display: block; }

/* ============================================================
   20. PYQ CARDS
   ============================================================ */
.pyq-card,
.pyq {
  background: var(--card);
  border: 1px solid var(--border);
  border-left: 4px solid var(--gold);
  border-radius: 16px;
  padding: 22px 24px;
  margin-bottom: 14px;
  box-shadow: var(--shadow-sm);
  transition: all 0.25s ease;
  position: relative;
  overflow: hidden;
}

.pyq-card:hover,
.pyq:hover {
  transform: translateX(4px);
  box-shadow: var(--shadow-md);
}

.pyq-year,
.yr {
  display: inline-block;
  font-family: var(--ui);
  font-size: 10.5px;
  font-weight: 800;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: #fff;
  background: linear-gradient(135deg, var(--gold), var(--red));
  padding: 4px 11px;
  border-radius: 999px;
  margin-bottom: 10px;
}

.pyq-q {
  font-weight: 600;
  margin-bottom: 10px;
  color: var(--ink);
  line-height: 1.6;
}

/* ============================================================
   21. NCERT ANSWER CARDS (Chapter 2 style)
   ============================================================ */
.answer-section { margin-top: 10px; }

.answer-hero {
  background: linear-gradient(135deg, rgba(5, 150, 105, 0.06), rgba(8, 145, 178, 0.05));
  border: 1.5px solid rgba(5, 150, 105, 0.16);
  border-radius: 18px;
  padding: 24px 26px;
  margin-bottom: 22px;
  box-shadow: var(--shadow-sm);
}

.answer-hero h3 {
  font-family: var(--serif);
  font-size: 1.5rem;
  margin-bottom: 8px;
  color: var(--ink);
}

.answer-hero p {
  color: var(--muted);
  margin: 0;
}

/* Revision strip (Chapter 2) */
.revision-strip {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-top: 14px;
}

.revision-chip {
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 700;
  padding: 6px 14px;
  border-radius: 999px;
  background: rgba(5, 150, 105, 0.1);
  color: var(--green);
  border: 1px solid rgba(5, 150, 105, 0.2);
}

html.dark .revision-chip {
  background: rgba(34, 197, 94, 0.15);
  color: #86efac;
  border-color: rgba(34, 197, 94, 0.3);
}

.answer-group {
  margin: 24px 0 12px;
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  letter-spacing: 0.15em;
  text-transform: uppercase;
  color: var(--green);
}

.answer-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 16px;
}

.answer-card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 16px;
  padding: 22px 24px;
  box-shadow: var(--shadow-sm);
  transition: all 0.25s ease;
}

.answer-card:hover { box-shadow: var(--shadow-md); }

.answer-title {
  display: flex;
  gap: 10px;
  align-items: flex-start;
  margin-bottom: 12px;
}

.answer-no {
  width: 30px;
  height: 30px;
  display: grid;
  place-items: center;
  border-radius: 10px;
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  font-family: var(--ui);
  font-weight: 800;
  font-size: 12px;
  flex: none;
}

.answer-title strong {
  font-family: var(--ui);
  font-size: 14px;
  line-height: 1.55;
  color: var(--ink);
}

.answer-body {
  display: none;
  padding: 20px 22px;
  border-top: 1px dashed rgba(5, 150, 105, 0.25);
  background: #fefeff;
  animation: slideDown 0.3s ease;
  color: #334155;
  font-size: 15px;
  line-height: 1.72;
  margin: 4px -4px -4px;
  border-radius: 0 0 12px 12px;
}

.answer-card.open .answer-body { display: block; }

.answer-key {
  display: inline-block;
  margin: 0 0 8px;
  padding: 5px 10px;
  border-radius: 999px;
  background: rgba(5, 150, 105, 0.09);
  color: var(--green);
  border: 1px solid rgba(5, 150, 105, 0.18);
  font-family: var(--ui);
  font-weight: 800;
  font-size: 11px;
  text-transform: uppercase;
  letter-spacing: 0.08em;
}

.answer-table-wrap {
  overflow: auto;
  border: 1px solid var(--border);
  border-radius: 14px;
  margin-top: 12px;
}

.answer-table {
  width: 100%;
  min-width: 620px;
  border-collapse: collapse;
  font-family: var(--ui);
  font-size: 13px;
  background: #fff;
}

.answer-table th,
.answer-table td {
  padding: 11px 12px;
  border-bottom: 1px solid var(--border);
  text-align: left;
  vertical-align: top;
}

.answer-table th {
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  font-weight: 700;
}

/* ===== NEW (FIX): Answer controls — JS-generated Show Answer button wrapper ===== */
.answer-controls {
  margin: 10px 0 4px;
}

/* ===== NEW (FIX): True/False interactive controls (Chapter 2) ===== */
.tf-controls {
  display: flex;
  gap: 20px;
  align-items: center;
  margin-top: 16px;
  flex-wrap: wrap;
}

.tf-controls label {
  display: flex;
  align-items: center;
  gap: 8px;
  font-family: var(--ui);
  font-size: 15px;
  font-weight: 600;
  cursor: pointer;
  color: var(--ink);
}

.tf-controls label input {
  accent-color: var(--green);
  cursor: pointer;
  width: 16px;
  height: 16px;
}

.tf-controls button {
  margin-left: auto;
  padding: 6px 18px;
  font-size: 13px;
}

html.dark .tf-controls label {
  color: var(--ink);
}

/* ===== NEW (FIX): True/False feedback message ===== */
.tf-feedback {
  display: none;
  margin-top: 16px;
  padding: 14px 18px;
  border-radius: 12px;
  font-family: var(--ui);
  font-size: 14px;
  line-height: 1.6;
}

.tf-feedback.correct,
.tf-feedback.wrong {
  display: block;
}

.tf-feedback.correct {
  background: rgba(5, 150, 105, 0.08);
  border-left: 4px solid var(--green);
  color: #065f46;
}

.tf-feedback.wrong {
  background: rgba(220, 38, 38, 0.08);
  border-left: 4px solid var(--red);
  color: #991b1b;
}

html.dark .tf-feedback.correct {
  background: rgba(34, 197, 94, 0.15);
  color: #86efac;
}

html.dark .tf-feedback.wrong {
  background: rgba(248, 113, 113, 0.15);
  color: #fca5a5;
}

@media (max-width: 480px) {
  .tf-controls button { margin-left: 0; }
}

/* True/False answer cards (older format) */
.tf-result {
  display: block;
  margin: 0 0 14px;
  padding: 13px 15px;
  border-radius: 12px;
  background: linear-gradient(135deg, rgba(5, 150, 105, 0.06), rgba(8, 145, 178, 0.04));
  border: 1px solid rgba(5, 150, 105, 0.16);
}

.tf-result .label {
  display: block;
  font-family: var(--ui);
  font-size: 10px;
  font-weight: 900;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--muted);
  margin-bottom: 5px;
}

.tf-result strong {
  display: block;
  font-family: var(--ui);
  font-size: 15px;
  font-weight: 900;
}

.tf-result.true strong { color: #047857; }
.tf-result.false strong { color: #b91c1c; }

.tf-correction {
  display: block;
  padding: 14px 15px;
  border-radius: 12px;
  background: #f8fafc;
  border: 1px solid rgba(15, 23, 42, 0.08);
}

.tf-correction span {
  display: block;
  margin-bottom: 5px;
  font-family: var(--ui);
  font-size: 10px;
  font-weight: 900;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--green);
}

.tf-correction strong {
  display: block;
  font-family: var(--body);
  font-size: 15px;
  line-height: 1.7;
  color: #334155;
  font-weight: 700;
}

/* ============================================================
   22. IMAGE / DIAGRAM FRAMES
   ============================================================ */
.diagram {
  margin: 22px 0;
  padding: 18px;
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 18px;
  box-shadow: var(--shadow-sm);
}

.diagram header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 10px;
  margin-bottom: 12px;
}

.diagram header h3 { margin: 0; }
.diagram header small {
  color: var(--muted);
  font-family: var(--ui);
}

.image-frame {
  position: relative;
  min-height: 320px;
  border-radius: 14px;
  background: #f8fafc;
  border: 1px dashed rgba(15, 23, 42, 0.16);
  overflow: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
}

.diagram-img {
  display: block;
  width: 100%;
  height: auto;
  max-height: 720px;
  object-fit: contain;
  background: #fff;
}

.image-placeholder {
  display: none;
  padding: 40px;
  text-align: center;
  color: #64748b;
  font-family: var(--ui);
}

.image-placeholder strong {
  display: block;
  color: #334155;
  font-size: 15px;
  margin-bottom: 6px;
}

.image-frame.missing .diagram-img { display: none; }
.image-frame.missing .image-placeholder { display: block; }
.image-frame.has-image .image-placeholder { display: none !important; }
.image-frame.has-image .diagram-img { display: block !important; }

.diagram-note {
  margin-top: 12px;
  color: #334155;
  font-size: 0.95rem;
}

.diagram-note strong { color: var(--ink); }

/* ============================================================
   23. LEGACY-SPECIFIC: NCERT Q, SUMMARY, FLASHCARDS
   ============================================================ */
.ncert-q {
  background: var(--card);
  border: 1.5px solid rgba(5, 150, 105, 0.25);
  border-radius: 14px;
  padding: 18px 22px;
  margin: 14px 0;
  box-shadow: var(--shadow-sm);
}

.ncert-q .qnum {
  display: inline-block;
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  padding: 3px 10px;
  border-radius: 999px;
  margin-bottom: 10px;
  letter-spacing: 0.06em;
}

.ncert-q .qtext {
  font-weight: 600;
  color: var(--ink);
  margin-bottom: 10px;
  line-height: 1.7;
}

.ncert-q details {
  margin-top: 12px;
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 12px 16px;
  background: rgba(5, 150, 105, 0.03);
}

.ncert-q summary {
  cursor: pointer;
  font-family: var(--ui);
  font-size: 13px;
  font-weight: 700;
  color: var(--green);
  user-select: none;
  list-style: none;
}

.ncert-q summary::before {
  content: "▶ ";
  font-size: 10px;
  margin-right: 4px;
  display: inline-block;
  transition: transform 0.2s;
}

.ncert-q details[open] summary::before { transform: rotate(90deg); }

.ncert-q details .sol {
  margin-top: 12px;
  padding-top: 12px;
  border-top: 1px dashed var(--border);
  font-size: 15px;
  line-height: 1.8;
  color: var(--ink);
}

.summary-box {
  background: linear-gradient(135deg, #dcfce7, #d1fae5);
  border: 2px solid rgba(5, 150, 105, 0.3);
  border-radius: 18px;
  padding: 28px;
  margin: 24px 0;
  box-shadow: var(--shadow-sm);
}

.summary-box h3 {
  font-family: var(--serif);
  font-size: 20px;
  font-weight: 800;
  color: var(--green-d);
  margin-bottom: 18px;
  display: flex;
  align-items: center;
  gap: 10px;
}

.summary-box h3::before {
  content: "📌";
  font-size: 24px;
}

.summary-box ol { padding-left: 28px; }

.summary-box li {
  font-size: 15.5px;
  line-height: 1.85;
  margin-bottom: 10px;
  color: var(--ink);
  padding-left: 8px;
}

.summary-box li strong {
  color: var(--green-d);
  font-weight: 800;
}

.exam-q-box {
  background: var(--card);
  border: 1.5px solid rgba(5, 150, 105, 0.25);
  border-radius: 18px;
  padding: 26px;
  margin: 20px 0;
  box-shadow: var(--shadow-sm);
}

.exam-q-box h4 {
  font-family: var(--ui);
  font-size: 16px;
  font-weight: 800;
  color: var(--green);
  margin: 18px 0 12px;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  padding-bottom: 6px;
  border-bottom: 2px solid rgba(5, 150, 105, 0.2);
}

.exam-q-box h4:first-child { margin-top: 0; }
.exam-q-box ol { padding-left: 26px; }

.exam-q-box li {
  font-size: 15.5px;
  line-height: 1.8;
  margin-bottom: 10px;
  color: var(--ink);
}

.flashcard-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  gap: 16px;
  margin: 20px 0;
}

.flashcard-static {
  background: var(--card);
  border: 2px solid rgba(5, 150, 105, 0.3);
  border-radius: 14px;
  padding: 20px 18px;
  text-align: center;
  box-shadow: var(--shadow-sm);
  transition: 0.2s;
  min-height: 130px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  gap: 10px;
}

.flashcard-static:hover {
  transform: translateY(-3px);
  box-shadow: 0 6px 20px rgba(5, 150, 105, 0.18);
  border-color: var(--green);
}

.flashcard-static .fc-title {
  font-family: var(--ui);
  font-size: 13.5px;
  font-weight: 700;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.08em;
  line-height: 1.4;
}

.flashcard-static .fc-formula {
  font-family: "Cambria Math", Georgia, serif;
  font-size: 15px;
  font-weight: 700;
  color: var(--green);
  padding: 10px 12px;
  background: linear-gradient(135deg, rgba(5, 150, 105, 0.08), rgba(8, 145, 178, 0.05));
  border-radius: 8px;
  line-height: 1.5;
}

/* Formula sheet */
.formula-sheet {
  background: var(--card);
  border: 1.5px solid rgba(5, 150, 105, 0.25);
  border-radius: 18px;
  padding: 28px;
  margin: 24px 0;
  box-shadow: var(--shadow-sm);
}

.formula-sheet h3 {
  font-family: var(--serif);
  font-size: 18px;
  font-weight: 800;
  color: var(--green);
  margin-bottom: 20px;
  text-align: center;
}

.formula-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 14px;
}

.formula-item {
  background: var(--bg-alt);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 14px 16px;
  transition: 0.2s;
}

.formula-item:hover {
  transform: translateY(-2px);
  box-shadow: 0 4px 16px rgba(5, 150, 105, 0.15);
}

.formula-item .ftitle {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 800;
  color: var(--green);
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-bottom: 6px;
}

.formula-item .fmath {
  font-family: "Cambria Math", Georgia, serif;
  font-size: 15px;
  font-weight: 600;
  color: var(--ink);
  line-height: 1.5;
}

/* Notation */
.notation {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: 14px;
  padding: 20px;
  margin: 20px 0;
}

.notation h4 {
  font-family: var(--ui);
  font-size: 13px;
  font-weight: 800;
  color: var(--green);
  text-transform: uppercase;
  letter-spacing: 0.12em;
  margin-bottom: 12px;
}

.notation table {
  min-width: 100%;
  font-size: 14px;
}

.notation td {
  padding: 8px 12px;
  border-bottom: 1px dashed var(--border);
}

.notation td:first-child {
  font-family: var(--ui);
  font-weight: 800;
  color: var(--green);
  width: 80px;
}

/* ============================================================
   24. FLOW DIAGRAMS
   ============================================================ */
.flow-wrap {
  overflow-x: auto;
  margin: 20px 0;
  padding: 10px 0;
}

.flow-svg {
  min-width: 700px;
  width: 100%;
  height: auto;
}

.flow-node {
  fill: #fff;
  stroke: var(--green);
  stroke-width: 2;
}

.flow-node-text {
  fill: var(--green-d);
  font-weight: 700;
  font-family: Inter, Arial;
  font-size: 11px;
  text-anchor: middle;
}

.flow-arrow {
  stroke: var(--green);
  stroke-width: 2;
  fill: none;
}

@keyframes dashFlow {
  to { stroke-dashoffset: -40; }
}

.flow-line {
  stroke-dasharray: 6 10;
  animation: dashFlow 2s linear infinite;
}

/* ============================================================
   25. RELATED LINKS & CHAPTER NAV
   ============================================================ */
.related-links {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 12px;
  margin: 20px 0;
}

.related-links a {
  display: flex;
  align-items: center;
  gap: 10px;
  background: var(--card);
  border: 1.5px solid var(--border);
  border-radius: 12px;
  padding: 14px 18px;
  text-decoration: none;
  color: var(--ink);
  transition: 0.2s;
  font-family: var(--ui);
  font-size: 14px;
  font-weight: 600;
}

.related-links a:hover {
  transform: translateY(-2px);
  border-color: var(--green);
  box-shadow: var(--shadow-sm);
}

.related-links .ico { font-size: 22px; }

.chapter-nav {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 16px;
  margin: 40px auto 20px;
  max-width: 920px;
  padding: 0 24px;
}

.chapter-nav a {
  display: flex;
  flex-direction: column;
  gap: 4px;
  background: var(--card);
  border: 1.5px solid var(--border);
  border-radius: 14px;
  padding: 16px 20px;
  text-decoration: none;
  color: var(--ink);
  transition: 0.2s;
}

.chapter-nav a:hover {
  transform: translateY(-2px);
  border-color: var(--green);
  box-shadow: var(--shadow-sm);
}

.chapter-nav .nav-dir {
  font-family: var(--ui);
  font-size: 11px;
  font-weight: 700;
  color: var(--green);
  text-transform: uppercase;
  letter-spacing: 0.12em;
}

.chapter-nav .nav-title {
  font-family: var(--ui);
  font-size: 15px;
  font-weight: 700;
}

.chapter-nav .next { text-align: right; }

/* ============================================================
   26. FOOTER
   ============================================================ */
footer {
  background: #0f172a;
  color: #cbd5e1;
  padding: 56px 24px 32px;
  margin-top: 60px;
  font-size: 14px;
  font-family: var(--ui);
}

.footer-inner,
.fi {
  max-width: none;
  margin: 0 auto;
  display: grid;
  grid-template-columns: 1.4fr 1fr 1fr 1fr 1fr;
  gap: 36px;
  padding-bottom: 32px;
  border-bottom: 1px solid rgba(255, 255, 255, 0.08);
}

.f-brand-wrap {
  display: flex;
  align-items: center;
  gap: 12px;
  margin-bottom: 14px;
}

.f-logo {
  width: 48px;
  height: 48px;
  font-size: 24px;
  border-radius: 12px;
}

.f-brand {
  font-family: var(--ui);
  font-size: 1.3rem;
  font-weight: 800;
  color: #f8fafc;
  letter-spacing: -0.02em;
}

.f-brand .accent { color: var(--cyan); }

.f-tag {
  font-family: var(--ui);
  font-size: 10px;
  font-weight: 600;
  letter-spacing: 0.2em;
  text-transform: uppercase;
  color: #94a3b8;
}

.f-col p {
  font-size: 14px;
  line-height: 1.7;
  margin-bottom: 8px;
}

.f-col h5,
footer h5 {
  font-family: var(--ui);
  font-size: 12px;
  font-weight: 700;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: #f8fafc;
  margin-bottom: 14px;
}

.f-col a,
footer a {
  display: block;
  font-family: var(--ui);
  font-size: 14px;
  color: #cbd5e1;
  text-decoration: none;
  padding: 4px 0;
  transition: color 0.2s ease;
}

.f-col a:hover,
footer a:hover { color: var(--cyan); }

/* ===== NEW (FIX): Footer quote — used in all 16 chapters ===== */
p.f-quote {
  font-style: italic;
  font-size: 13.5px;
  color: #94a3b8;
  margin-top: 10px;
  padding: 8px 12px;
  border-left: 3px solid var(--green-l);
  background: rgba(255, 255, 255, 0.04);
  border-radius: 0 8px 8px 0;
  line-height: 1.6;
}

.f-bottom,
.fb {
  max-width: 1140px;
  margin: 22px auto 0;
  text-align: center;
  font-family: var(--ui);
  font-size: 12.5px;
  color: #94a3b8;
}

/* ============================================================
   27. BACK TO TOP
   ============================================================ */
#backTop {
  position: fixed;
  bottom: 24px;
  right: 24px;
  width: 48px;
  height: 48px;
  border-radius: 50%;
  background: linear-gradient(135deg, var(--green), var(--cyan));
  color: #fff;
  border: none;
  font-size: 20px;
  cursor: pointer;
  opacity: 0;
  visibility: hidden;
  transition: 0.3s;
  box-shadow: 0 6px 20px rgba(5, 150, 105, 0.5);
  z-index: 800;
  font-weight: 900;
}

#backTop.show { opacity: 1; visibility: visible; }

#backTop:hover {
  transform: translateY(-3px);
  box-shadow: 0 8px 26px rgba(5, 150, 105, 0.6);
}

/* ============================================================
   28. UTILITY CLASSES
   ============================================================ */
.source-note {
  font-family: var(--ui);
  font-size: 0.84rem;
  color: var(--muted);
}

.mini-list {
  margin: 8px 0 0;
  padding-left: 20px;
}

.check-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 8px;
}

.check-grid .check {
  padding: 10px 12px;
  border: 1px solid var(--border);
  border-radius: 10px;
  background: var(--card);
}

.grid {
  display: grid;
  gap: 18px;
}

.grid.two { grid-template-columns: repeat(2, minmax(0, 1fr)); }
.grid.three { grid-template-columns: repeat(3, minmax(0, 1fr)); }

/* ============================================================
   29. ANIMATIONS
   ============================================================ */
@keyframes particleFloat {
  0% { transform: translate(0, 0); opacity: 0; }
  10% { opacity: 0.9; }
  90% { opacity: 0.9; }
  100% { transform: translate(var(--dx), var(--dy)); opacity: 0; }
}

.hero-particles span {
  position: absolute;
  width: 6px;
  height: 6px;
  border-radius: 50%;
  background: radial-gradient(circle, #4ade80, #16a34a);
  box-shadow: 0 0 12px rgba(74, 222, 128, 0.9);
  animation: particleFloat 6s ease-in-out infinite;
}

.hero-particles span:nth-child(1) { top: 20%; left: 12%; --dx: 60px; --dy: -40px; }
.hero-particles span:nth-child(2) { top: 60%; left: 8%; --dx: 40px; --dy: -80px; animation-delay: 1.2s; }
.hero-particles span:nth-child(3) { top: 30%; right: 15%; --dx: -50px; --dy: 50px; animation-delay: 0.6s; }
.hero-particles span:nth-child(4) { top: 75%; right: 10%; --dx: -60px; --dy: -30px; animation-delay: 2s; }
.hero-particles span:nth-child(5) { top: 15%; left: 50%; --dx: 30px; --dy: 60px; animation-delay: 1.6s; }
.hero-particles span:nth-child(6) { top: 80%; left: 40%; --dx: -30px; --dy: -50px; animation-delay: 0.4s; }

/* ============================================================
   30. RESPONSIVE
   ============================================================ */
@media (max-width: 960px) {
  .footer-inner, .fi {
    grid-template-columns: 1.2fr 1fr 1fr;
    gap: 28px;
  }
  .grid.three { grid-template-columns: 1fr 1fr; }
  .stats-inner, .si { grid-template-columns: repeat(2, 1fr); }
}

@media (max-width: 760px) {
  .answer-grid { grid-template-columns: 1fr; }
  .answer-hero { padding: 20px; }
  .answer-card { padding: 18px; }
  .answer-body { padding: 18px 16px; }
}

@media (max-width: 720px) {
  body { font-size: 16px; }

  .nav-inner, .ni { padding: 10px 14px; }
  .brand-tag { display: none; }
  .brand-logo, .logo { width: 38px; height: 38px; font-size: 18px; }
  .brand-name, .bn { font-size: 17px; }
  .nav-links a, .nl a { padding: 6px 10px; font-size: 12px; }

  .hero { padding: 44px 18px 54px; }
  .section, .topic { padding: 52px 18px; }
  .stats-inner, .si { padding: 18px 16px; gap: 12px; }
  .stat-item, .si > div { border-right: 0; }

  .cat-nav { padding: 10px 14px; }
  .cat-tab { padding: 7px 14px; font-size: 12px; }

  .content-card, .card, .cc { padding: 22px 20px; }
  .mcq-form, .quiz, .qf { padding: 20px 16px; }

  .opts, .o { padding-left: 0; }
  .explain, .ex { margin-left: 0; }

  .score { margin-left: 0; width: 100%; text-align: center; }
  .mcq-actions .btn, .act .btn { flex: 1; justify-content: center; }

  .footer-inner, .fi { grid-template-columns: 1fr 1fr; gap: 24px; }

  .show-answer-body, .sa .b { padding: 18px 16px; }
  .ar-card { padding: 18px 16px; }
  .viz-box, .viz { padding: 14px 10px; }
  .viz-box .bio-svg, .viz svg { min-width: 680px; }
  .viz-caption, .cap { font-size: 12.5px; }

  .calc-results { grid-template-columns: 1fr; }
  .calc-grid { grid-template-columns: 1fr; }
  .chapter-nav { grid-template-columns: 1fr; }
  .chapter-nav .next { text-align: left; }
  .summary-box { padding: 20px 18px; }
  .chapter-nav-top { display: none; }
  .image-frame { min-height: 240px; }

  #backTop { bottom: 16px; right: 16px; width: 44px; height: 44px; }

  .coverage-strip { padding: 12px 14px; }
  .revision-strip { gap: 6px; }
  .revision-chip { font-size: 11px; padding: 5px 12px; }
}

@media (max-width: 520px) {
  body { font-size: 16px; }
  .nav-links a, .nl a { padding: 5px 9px; font-size: 11.5px; }
  .footer-inner, .fi { grid-template-columns: 1fr; gap: 24px; }
  .content-card h3, .cc h3 { font-size: 1.1rem; flex-wrap: wrap; }
  .topic-head { gap: 10px; }
  .num { width: 48px; height: 48px; }
  .stats-inner, .si { grid-template-columns: 1fr 1fr; }
  .image-frame { min-height: 200px; }
}

/* ============================================================
   31. PRINT
   ============================================================ */
@media print {
  .nav, .cat-nav, .progress-track, .pt, .hero-meta,
  .stats-band, .stats, .btn, #backTop, .theme-btn,
  .print-btn, .act, .sa button, .viz svg,
  .read-btn, .sec-actions, .chapter-nav,
  .related-links, .hero-particles, .bc {
    display: none !important;
  }

  .viz, .viz-box {
    overflow: visible;
    background: #fff;
    border: 1px solid #ddd;
    padding: 8px;
  }

  .viz-box .bio-svg, .viz svg {
    min-width: 0;
    width: 100%;
  }

  body {
    font-size: 11pt;
    line-height: 1.5;
    background: #fff;
    color: #000;
  }

  .hero {
    padding: 20px;
    border: 0;
    background: #fff;
    border-bottom: 2px solid #333;
  }

  .section, .topic {
    padding: 24px 0;
    page-break-inside: avoid;
  }

  .content-card, .card, .cc, .diagram {
    box-shadow: none;
    border: 1px solid #ccc;
    page-break-inside: avoid;
  }

  .image-frame { break-inside: avoid; }

  .formula-sheet {
    border: 2px solid #16a34a;
    page-break-inside: avoid;
  }

  .real, .mistake, .why, .analogy, .try, .trap,
  .takeaway, .def, .note, .exam, .did-you-know,
  .mnemonic-box, .flashcard-static, .callout {
    background: #f8f8f8 !important;
    page-break-inside: avoid;
  }

  .q, .ex, .ncert-q, .ans { page-break-inside: avoid; }

  .sa .b, .show-answer-body {
    display: block !important;
    border-top: none;
    padding: 0;
  }

  .answer-body { display: block !important; }

  .footer-inner, .fi { display: none; }

  footer {
    background: #fff;
    color: #333;
    border-top: 1px solid #ccc;
    padding: 20px;
    margin-top: 20px;
  }
}