:root {
  --bg: #ffffff;
  --bg-soft: #f6f7f9;
  --bg-raise: #ffffff;
  --text: #1c2330;
  --text-soft: #55617a;
  --line: #e3e7ee;
  --accent: #3956d8;
  --accent-soft: #eaeefc;
  --accent-text: #ffffff;
  --good: #1a7f4b;
  --good-soft: #e2f4ea;
  --bad: #b3382f;
  --bad-soft: #fbe9e7;
  --core: #8a5a00;
  --core-soft: #fdf3dc;
  --shadow: 0 8px 24px rgba(28, 35, 48, 0.08);
  --radius: 10px;
  --bar-in: #3956d8;
  --bar-out: #8494b8;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #12151c;
    --bg-soft: #181c25;
    --bg-raise: #1e2330;
    --text: #e6eaf2;
    --text-soft: #9aa5bc;
    --line: #2a3140;
    --accent: #7d95f5;
    --accent-soft: #232c4a;
    --accent-text: #0f1320;
    --good: #4cc98a;
    --good-soft: #16311f;
    --bad: #ef8a7f;
    --bad-soft: #3a1d1a;
    --core: #e8b64c;
    --core-soft: #33290f;
    --shadow: 0 8px 24px rgba(0, 0, 0, 0.4);
    --bar-in: #7d95f5;
    --bar-out: #55617f;
  }
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  font-size: 17px;
}

h1, h2, h3, h4 { line-height: 1.25; }

h1 { font-size: 1.9rem; margin: 0 0 0.4rem; }

h2 { font-size: 1.45rem; margin: 2.6rem 0 0.8rem; padding-top: 0.5rem; }

h3 { font-size: 1.12rem; margin: 1.6rem 0 0.5rem; }

p { margin: 0.8rem 0; }

a { color: var(--accent); text-decoration: none; }

a:hover { text-decoration: underline; }

code {
  background: var(--bg-soft);
  border: 1px solid var(--line);
  border-radius: 5px;
  padding: 0.08em 0.35em;
  font-size: 0.9em;
}

.layout {
  display: grid;
  grid-template-columns: 250px minmax(0, 1fr);
  gap: 2.5rem;
  max-width: 1100px;
  margin: 0 auto;
  padding: 0 1.2rem 4rem;
}

.toc {
  position: sticky;
  top: 0;
  align-self: start;
  max-height: 100vh;
  overflow-y: auto;
  padding: 1.6rem 0.4rem 2rem 0;
  font-size: 0.88rem;
}

.toc nav { border-left: 2px solid var(--line); }

.toc a {
  display: block;
  padding: 0.32rem 0.9rem;
  color: var(--text-soft);
  border-left: 2px solid transparent;
  margin-left: -2px;
}

.toc a:hover { color: var(--accent); text-decoration: none; }

.toc a.active {
  color: var(--accent);
  border-left-color: var(--accent);
  background: var(--accent-soft);
}

.toc .toc-title {
  font-weight: 600;
  color: var(--text);
  margin: 0 0 0.6rem;
  font-size: 0.82rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
}

main { min-width: 0; padding-top: 1.6rem; }

.hero {
  border-bottom: 1px solid var(--line);
  padding: 1rem 0 1.4rem;
  margin-bottom: 1rem;
}

.hero .kicker {
  color: var(--accent);
  font-weight: 600;
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  margin: 0;
}

.hero .meta { color: var(--text-soft); font-size: 0.92rem; margin: 0.3rem 0 0; }

.core {
  background: var(--core-soft);
  border-left: 4px solid var(--core);
  border-radius: 0 var(--radius) var(--radius) 0;
  padding: 0.8rem 1.1rem;
  margin: 1.2rem 0;
}

.core::before {
  content: "★ Core to learn";
  display: block;
  font-weight: 700;
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--core);
  margin-bottom: 0.25rem;
}

.bigpic {
  background: var(--accent-soft);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 0.9rem 1.1rem;
  margin: 1.2rem 0;
}

.bigpic::before {
  content: "🔭 Bigger picture";
  display: block;
  font-weight: 700;
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--accent);
  margin-bottom: 0.25rem;
}

details.deeper {
  border: 1px solid var(--line);
  border-radius: var(--radius);
  margin: 1.1rem 0;
  background: var(--bg-soft);
}

details.deeper > summary {
  cursor: pointer;
  padding: 0.7rem 1.1rem;
  font-weight: 600;
  color: var(--accent);
  list-style: none;
}

details.deeper > summary::before { content: "▸ "; }

details.deeper[open] > summary::before { content: "▾ "; }

details.deeper > summary:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-radius: var(--radius);
}

details.deeper > .deeper-body { padding: 0 1.1rem 0.9rem; }

details.deeper .pageref {
  font-size: 0.85rem;
  color: var(--text-soft);
  font-style: italic;
}

.term {
  border-bottom: 1.5px dotted var(--accent);
  cursor: help;
  font-weight: 500;
}

.term:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-radius: 3px;
}

.tooltip-pop {
  position: absolute;
  z-index: 50;
  max-width: 320px;
  background: var(--bg-raise);
  color: var(--text);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 0.7rem 0.9rem;
  font-size: 0.88rem;
  line-height: 1.5;
}

.tooltip-pop .tt-name { font-weight: 700; display: block; margin-bottom: 0.2rem; }

.tooltip-pop a { display: inline-block; margin-top: 0.35rem; font-weight: 600; }

figure.diagram {
  margin: 1.4rem 0;
  padding: 1rem;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--bg-soft);
  overflow-x: auto;
}

figure.diagram figcaption {
  font-size: 0.85rem;
  color: var(--text-soft);
  margin-top: 0.6rem;
}

figure.diagram svg { max-width: 100%; height: auto; display: block; margin: 0 auto; }

.interactive {
  border: 1px solid var(--accent);
  border-radius: var(--radius);
  padding: 1rem 1.1rem;
  margin: 1.4rem 0;
  background: var(--bg-raise);
  box-shadow: var(--shadow);
}

.interactive .int-title {
  font-weight: 700;
  margin: 0 0 0.6rem;
  color: var(--accent);
  font-size: 0.95rem;
}

.controls {
  display: flex;
  flex-wrap: wrap;
  gap: 0.8rem 1.4rem;
  align-items: center;
  margin: 0.6rem 0;
}

.controls label { font-size: 0.86rem; color: var(--text-soft); }

.controls input[type="range"] { vertical-align: middle; accent-color: var(--accent); }

.controls output { font-variant-numeric: tabular-nums; font-weight: 600; }

.btn {
  display: inline-block;
  background: var(--accent);
  color: var(--accent-text);
  border: none;
  border-radius: 7px;
  padding: 0.45rem 1rem;
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
  font-family: inherit;
}

.btn:hover { filter: brightness(1.08); }

.btn:focus-visible { outline: 2px solid var(--text); outline-offset: 2px; }

.btn.secondary {
  background: var(--bg-soft);
  color: var(--text);
  border: 1px solid var(--line);
}

.btn:disabled { opacity: 0.45; cursor: not-allowed; }

.stepnote {
  min-height: 3.2em;
  font-size: 0.9rem;
  background: var(--bg-soft);
  border-radius: 7px;
  padding: 0.5rem 0.8rem;
  margin-top: 0.6rem;
}

.quiz-panel {
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 1.2rem 1.3rem;
  margin: 1rem 0 2rem;
  background: var(--bg-raise);
  box-shadow: var(--shadow);
}

.quiz-progress {
  font-size: 0.88rem;
  color: var(--text-soft);
  margin-bottom: 0.8rem;
  font-variant-numeric: tabular-nums;
}

.quiz-q { font-weight: 600; font-size: 1.05rem; margin: 0.2rem 0 0.9rem; }

.quiz-opts { display: grid; gap: 0.55rem; margin: 0; padding: 0; list-style: none; }

.quiz-opt {
  text-align: left;
  width: 100%;
  background: var(--bg-soft);
  border: 1.5px solid var(--line);
  border-radius: 8px;
  padding: 0.6rem 0.9rem;
  font-size: 0.95rem;
  font-family: inherit;
  color: var(--text);
  cursor: pointer;
}

.quiz-opt:hover { border-color: var(--accent); }

.quiz-opt:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }

.quiz-opt.correct { border-color: var(--good); background: var(--good-soft); }

.quiz-opt.wrong { border-color: var(--bad); background: var(--bad-soft); }

.quiz-opt:disabled { cursor: default; }

.quiz-explain {
  border-left: 4px solid var(--good);
  background: var(--good-soft);
  border-radius: 0 8px 8px 0;
  padding: 0.7rem 1rem;
  font-size: 0.92rem;
  margin: 0.9rem 0;
}

.quiz-explain.wrong { border-left-color: var(--bad); background: var(--bad-soft); }

.quiz-done {
  text-align: center;
  padding: 1.4rem 0.5rem;
}

.quiz-done .big { font-size: 2.2rem; margin-bottom: 0.4rem; }

.badge {
  display: inline-block;
  background: var(--good-soft);
  color: var(--good);
  font-weight: 700;
  font-size: 0.78rem;
  border-radius: 999px;
  padding: 0.18rem 0.7rem;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.notice {
  background: var(--core-soft);
  border: 1px solid var(--core);
  color: var(--text);
  border-radius: var(--radius);
  padding: 0.6rem 1rem;
  font-size: 0.88rem;
  margin: 1rem 0;
}

.chapter-list { list-style: none; margin: 0; padding: 0; }

.chapter-list .part-h {
  margin: 1.8rem 0 0.6rem;
  font-size: 0.82rem;
  text-transform: uppercase;
  letter-spacing: 0.07em;
  color: var(--text-soft);
  font-weight: 700;
}

.chapter-list li.ch {
  display: flex;
  align-items: center;
  gap: 0.8rem;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 0.65rem 1rem;
  margin: 0.45rem 0;
  background: var(--bg-raise);
}

.chapter-list li.ch .num {
  color: var(--text-soft);
  font-variant-numeric: tabular-nums;
  min-width: 2ch;
  text-align: right;
}

.chapter-list li.ch .title { flex: 1; }

.chapter-list li.ch.locked { opacity: 0.55; }

.chapter-list li.ch.locked .title { color: var(--text-soft); }

.footer-nav {
  border-top: 1px solid var(--line);
  margin-top: 3rem;
  padding-top: 1.2rem;
  display: flex;
  justify-content: space-between;
  font-size: 0.92rem;
  color: var(--text-soft);
}

.skip-link {
  position: absolute;
  left: -9999px;
  background: var(--accent);
  color: var(--accent-text);
  padding: 0.5rem 1rem;
  border-radius: 7px;
  z-index: 100;
}

.skip-link:focus { left: 0.8rem; top: 0.8rem; }

@media (max-width: 860px) {
  .layout { grid-template-columns: 1fr; gap: 0; }
  .toc { position: static; max-height: none; padding-top: 1rem; }
  body { font-size: 16px; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  * { animation-duration: 0.01ms !important; transition-duration: 0.01ms !important; }
}