:root {
  --bg: #0f1729;
  --bg-soft: #16213e;
  --card: #1b2a4a;
  --ink: #eef2ff;
  --ink-soft: #aab6d6;
  --accent: #4dd0e1;
  --accent-2: #7c5cff;
  --core: #4dd0e1;
  --peripheral: #ffb74d;
  --extras: #81c784;
  --line: rgba(255, 255, 255, 0.08);
  --radius: 16px;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: radial-gradient(1200px 600px at 50% -10%, #1c2b52 0%, var(--bg) 55%);
  color: var(--ink);
  line-height: 1.6;
  min-height: 100vh;
}

a { color: var(--accent); }

.wrap {
  max-width: 1080px;
  margin: 0 auto;
  padding: 32px 20px 64px;
}

/* ---------- Header ---------- */
.site-header { text-align: center; padding: 24px 0 8px; }
.site-header h1 {
  font-size: clamp(28px, 5vw, 46px);
  margin: 0 0 6px;
  letter-spacing: -0.5px;
  background: linear-gradient(90deg, var(--accent), var(--accent-2));
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}
.site-header p {
  color: var(--ink-soft);
  margin: 0 auto;
  max-width: 680px;
  font-size: 20px;
}

/* ---------- Hero 3D model ---------- */
.hero { margin: 26px 0 8px; }
.hero-viewer {
  aspect-ratio: 16 / 9;
  box-shadow: 0 18px 50px rgba(0, 0, 0, 0.45);
}
.hero-hint {
  text-align: center;
  font-size: 18px;
  font-weight: 600;
  color: var(--ink);
  margin: 14px 0 2px;
}
.hero .credit { text-align: center; }

.instructions {
  background: var(--bg-soft);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 18px 22px;
  margin: 28px 0 8px;
}
.instructions h2 { margin: 0 0 8px; font-size: 20px; }
.instructions ol { margin: 0; padding-left: 20px; color: var(--ink-soft); font-size: 17px; }
.instructions ol li { margin: 4px 0; }
.worksheet-link { margin: 14px 0 0; }
.worksheet-link a { font-weight: 600; }

/* ---------- Category sections ---------- */
.category-title {
  margin: 40px 0 14px;
  font-size: 14px;
  text-transform: uppercase;
  letter-spacing: 2px;
  color: var(--ink-soft);
  display: flex;
  align-items: center;
  gap: 10px;
}
.category-title::before {
  content: "";
  width: 10px; height: 10px;
  border-radius: 50%;
  background: var(--dot, var(--accent));
}

/* ---------- Card grid ---------- */
.grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));
  gap: 16px;
}
.card {
  display: block;
  background: var(--card);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 20px;
  text-decoration: none;
  color: var(--ink);
  transition: transform 0.15s ease, border-color 0.15s ease, box-shadow 0.15s ease;
  position: relative;
  overflow: hidden;
}
.card::after {
  content: "";
  position: absolute;
  top: 0; left: 0; right: 0;
  height: 4px;
  background: var(--tab, var(--accent));
}
.card:hover {
  transform: translateY(-4px);
  border-color: var(--tab, var(--accent));
  box-shadow: 0 12px 30px rgba(0, 0, 0, 0.35);
}
.card-top {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 10px;
  margin-bottom: 4px;
}
.card-icon {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 46px;
  height: 46px;
  border-radius: 12px;
  background: color-mix(in srgb, var(--tab, var(--accent)) 16%, transparent);
  color: var(--tab, var(--accent));
  flex: none;
}
.part-icon { width: 28px; height: 28px; display: block; }
.card .num {
  font-size: 13px;
  color: var(--ink-soft);
  font-variant-numeric: tabular-nums;
}
.card h3 { margin: 6px 0 6px; font-size: 21px; }
.card p { margin: 0; color: var(--ink-soft); font-size: 15px; }

.cat-Core { --tab: var(--core); --dot: var(--core); }
.cat-Peripheral { --tab: var(--peripheral); --dot: var(--peripheral); }
.cat-Cooling { --tab: var(--extras); --dot: var(--extras); }

/* ---------- Part page ---------- */
.topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  margin-bottom: 18px;
  flex-wrap: wrap;
}
.back {
  text-decoration: none;
  color: var(--ink-soft);
  font-size: 14px;
  border: 1px solid var(--line);
  padding: 8px 14px;
  border-radius: 999px;
  transition: border-color 0.15s, color 0.15s;
}
.back:hover { color: var(--ink); border-color: var(--accent); }

.pager { display: flex; gap: 8px; }
.pager a {
  text-decoration: none;
  color: var(--ink-soft);
  font-size: 14px;
  border: 1px solid var(--line);
  padding: 8px 14px;
  border-radius: 999px;
}
.pager a:hover { color: var(--ink); border-color: var(--accent); }
.pager a.disabled { opacity: 0.35; pointer-events: none; }

/* Big named Prev/Next buttons at the bottom of a part page */
.pager-bottom {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 14px;
  margin-top: 36px;
}
.pager-btn {
  display: flex;
  flex-direction: column;
  gap: 4px;
  text-decoration: none;
  background: var(--card);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 16px 20px;
  transition: transform 0.15s ease, border-color 0.15s ease, box-shadow 0.15s ease;
}
.pager-btn:hover {
  transform: translateY(-3px);
  border-color: var(--accent);
  box-shadow: 0 10px 24px rgba(0, 0, 0, 0.3);
}
.pager-btn .pager-dir { font-size: 13px; color: var(--ink-soft); }
.pager-btn .pager-name { font-size: 19px; font-weight: 600; color: var(--ink); }
#next { text-align: right; }
.pager-btn.disabled { opacity: 0.3; pointer-events: none; }

.badge {
  display: inline-block;
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: 1.5px;
  padding: 4px 12px;
  border-radius: 999px;
  background: var(--bg-soft);
  border: 1px solid var(--line);
  color: var(--tab, var(--accent));
}
.part-head-top {
  display: flex;
  align-items: center;
  gap: 14px;
  margin-bottom: 4px;
}
.part-head-icon {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 60px;
  height: 60px;
  border-radius: 16px;
  background: color-mix(in srgb, var(--tab, var(--accent)) 16%, transparent);
  color: var(--tab, var(--accent));
  flex: none;
}
.part-head-icon .part-icon { width: 36px; height: 36px; }
.progress {
  display: inline-block;
  margin-left: 8px;
  font-size: 13px;
  color: var(--ink-soft);
  font-variant-numeric: tabular-nums;
}
.part-header h1 { font-size: clamp(28px, 5vw, 42px); margin: 12px 0 4px; }
.part-header .tagline { color: var(--ink-soft); font-size: 18px; margin: 0 0 18px; }

/* Read-aloud button */
.listen-btn {
  display: inline-flex;
  align-items: center;
  gap: 9px;
  font: inherit;
  font-size: 17px;
  font-weight: 600;
  color: var(--tab, var(--accent));
  background: color-mix(in srgb, var(--tab, var(--accent)) 14%, transparent);
  border: 1.5px solid var(--tab, var(--accent));
  border-radius: 999px;
  padding: 11px 22px;
  cursor: pointer;
  margin-bottom: 22px;
  transition: background 0.15s, transform 0.1s;
}
.listen-btn:hover { background: color-mix(in srgb, var(--tab, var(--accent)) 24%, transparent); }
.listen-btn:active { transform: scale(0.97); }
.listen-btn .listen-icon { font-size: 19px; line-height: 1; }
.listen-btn.is-playing {
  background: var(--tab, var(--accent));
  color: #0b1120;
}

/* 3D viewer */
.viewer {
  position: relative;
  width: 100%;
  aspect-ratio: 16 / 10;
  background: #0b1120;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  overflow: hidden;
}
.viewer iframe { width: 100%; height: 100%; border: 0; display: block; }
.viewer-hint {
  font-size: 13px;
  color: var(--ink-soft);
  margin: 8px 2px 0;
}
.credit { font-size: 12px; color: var(--ink-soft); margin: 4px 2px 0; }

/* Info blocks */
.info { margin-top: 28px; display: grid; gap: 20px; }
.info-block {
  background: var(--card);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 20px 24px;
}
.info-block h2 {
  margin: 0 0 8px;
  font-size: 16px;
  text-transform: uppercase;
  letter-spacing: 1.5px;
  color: var(--tab, var(--accent));
}
.info-block p { margin: 0; }
.facts { margin: 0; padding-left: 20px; }
.facts li { margin: 6px 0; }
.analogy {
  font-style: italic;
  color: var(--ink);
  border-left: 3px solid var(--tab, var(--accent));
  padding-left: 14px;
}

.cat-Core { --tab: var(--core); }
.cat-Peripheral { --tab: var(--peripheral); }
.cat-Cooling { --tab: var(--extras); }

footer {
  text-align: center;
  color: var(--ink-soft);
  font-size: 13px;
  margin-top: 48px;
  padding-top: 20px;
  border-top: 1px solid var(--line);
}

@media (max-width: 520px) {
  .topbar { flex-direction: column; align-items: stretch; }
  .pager { justify-content: space-between; }
  .pager-bottom { grid-template-columns: 1fr; }
}