:root {
  --bg: #ffffff;
  --bg-alt: #f4f6fb;
  --text: #1c2333;
  --muted: #5b6475;
  --accent: #2f5bea;
  --accent-text: #ffffff;
  --border: #e2e6ef;
  --good: #1e7a4c;
  --bad: #b3261e;
  color-scheme: light dark;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0f1420;
    --bg-alt: #161d2d;
    --text: #e8ecf4;
    --muted: #a3acbf;
    --accent: #6d8dff;
    --accent-text: #0f1420;
    --border: #273047;
    --good: #5ccb8f;
    --bad: #ff8a80;
  }
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
}

section[id],
article[id] { scroll-margin-top: 80px; }

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

.skip-link {
  position: absolute;
  left: 16px;
  top: -48px;
  padding: 8px 16px;
  border-radius: 8px;
  background: var(--accent);
  color: var(--accent-text);
  font-weight: 600;
  z-index: 20;
}

.skip-link:focus { top: 12px; }

body {
  margin: 0;
  font-family: system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
  line-height: 1.6;
  color: var(--text);
  background: var(--bg);
}

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

.container {
  max-width: 1040px;
  margin: 0 auto;
  padding: 0 16px;
}

.site-header {
  background: var(--bg);
  border-bottom: 1px solid var(--border);
}

@media (min-width: 600px) {
  .site-header {
    position: sticky;
    top: 0;
    z-index: 10;
  }
}

.nav {
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 12px;
  padding-top: 16px;
  padding-bottom: 16px;
}

.logo {
  font-weight: 700;
  font-size: 1.25rem;
  color: var(--text);
  text-decoration: none;
}

.logo span { color: var(--accent); }

nav a {
  margin-left: 20px;
  color: var(--muted);
  text-decoration: none;
}

nav a:first-child { margin-left: 0; }
nav a:hover { color: var(--text); }

.hero {
  padding: 96px 0 80px;
  text-align: center;
}

.hero h1 {
  font-size: clamp(2rem, 6vw, 3.25rem);
  line-height: 1.15;
  margin: 0 0 16px;
}

.hero p {
  font-size: 1.15rem;
  color: var(--muted);
  max-width: 600px;
  margin: 0 auto 32px;
}

.btn {
  display: inline-block;
  padding: 12px 28px;
  border-radius: 8px;
  background: var(--accent);
  color: var(--accent-text);
  font-weight: 600;
  text-decoration: none;
}

.btn:hover { opacity: 0.9; }

.section { padding: 64px 0; }
.section.alt { background: var(--bg-alt); }
.section h2 { margin-top: 0; font-size: 1.75rem; }

.cards {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 20px;
}

.card {
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 24px;
  background: var(--bg);
  transition: border-color 0.2s, transform 0.2s;
}

.card:hover {
  border-color: var(--accent);
  transform: translateY(-2px);
}

@media (prefers-reduced-motion: reduce) {
  .card { transition: none; }
  .card:hover { transform: none; }
}

.card h3 { margin-top: 0; }
.card p { color: var(--muted); margin-bottom: 0; }

.site-footer {
  border-top: 1px solid var(--border);
  padding: 24px 0;
  color: var(--muted);
  font-size: 0.9rem;
}

nav a[aria-current="page"] { color: var(--text); font-weight: 600; }

.concept { max-width: 720px; }
.concept h1 {
  font-size: clamp(1.9rem, 5vw, 2.6rem);
  line-height: 1.2;
  margin: 0 0 12px;
}
.concept h2 { font-size: 1.6rem; line-height: 1.25; margin: 0 0 12px; }
.concept h3 { font-size: 1.2rem; margin: 32px 0 8px; }

.concept-block {
  margin-top: 56px;
  padding-top: 40px;
  border-top: 1px solid var(--border);
}

.toc {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
  gap: 12px;
  margin: 24px 0 0;
  padding: 0;
  list-style: none;
}

.toc a {
  display: block;
  height: 100%;
  padding: 14px 16px;
  border: 1px solid var(--border);
  border-radius: 12px;
  background: var(--bg);
  color: var(--text);
  text-decoration: none;
  transition: border-color 0.2s;
}

.toc a:hover { border-color: var(--accent); }
.toc strong { display: block; color: var(--accent); }
.toc span { display: block; color: var(--muted); font-size: 0.9rem; line-height: 1.4; }

.formula {
  display: inline-block;
  margin: 4px 0;
  padding: 6px 14px;
  border-radius: 8px;
  background: var(--bg-alt);
  font-weight: 600;
}

.eyebrow {
  margin: 0 0 8px;
  color: var(--accent);
  font-weight: 600;
  font-size: 0.85rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

.lead { font-size: 1.15rem; color: var(--muted); }

.figure {
  margin: 24px 0;
  padding: 16px;
  border: 1px solid var(--border);
  border-radius: 12px;
  background: var(--bg-alt);
  text-align: center;
}

.figure svg {
  display: block;
  width: 100%;
  max-width: 360px;
  height: auto;
  margin: 0 auto;
}

.figure figcaption {
  margin-top: 8px;
  color: var(--muted);
  font-size: 0.95rem;
}

.btn-small {
  margin-top: 12px;
  padding: 6px 14px;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--bg);
  color: var(--text);
  font: inherit;
  font-size: 0.9rem;
  cursor: pointer;
}

.btn-small:hover { border-color: var(--accent); }

.g-earth { fill: var(--accent); fill-opacity: 0.18; stroke: var(--accent); stroke-width: 1.5; }
.g-mountain { fill: var(--muted); }
.g-path { fill: none; stroke: var(--muted); stroke-width: 1.2; stroke-dasharray: 3 4; }
.g-grid { fill: none; stroke: var(--muted); stroke-opacity: 0.5; stroke-width: 1.2; }
.g-ball { fill: var(--text); }
.g-ball-orbit { fill: var(--accent); }
.g-label { fill: var(--muted); font-size: 12px; }
.g-label-accent { fill: var(--accent); font-weight: 600; }
.g-arrow { fill: var(--muted); }
.g-route { fill: none; stroke: var(--border); stroke-width: 3; }
.g-trail { fill: none; stroke: var(--accent); stroke-width: 3; stroke-linecap: round; }
.g-tick { stroke: var(--muted); stroke-width: 1; }
.g-ghost { fill: var(--muted); fill-opacity: 0.25; }

.recap {
  margin-top: 40px;
  padding: 20px 24px;
  border-left: 4px solid var(--accent);
  border-radius: 0 12px 12px 0;
  background: var(--bg-alt);
}

.recap h2,
.recap h3 { margin-top: 0; }
.recap p { margin-bottom: 0; }

.exam-course {
  display: block;
  color: var(--text);
  text-decoration: none;
}

.exam-courses { margin-top: 24px; }
.exam-course h2 { margin: 0 0 8px; color: var(--accent); }

.exam-meta { color: var(--muted); font-size: 0.95rem; margin: 4px 0 12px; }
.exam-note { color: var(--muted); font-size: 0.9rem; margin-top: 40px; }

.exam-summary {
  margin: 24px 0 8px;
  padding: 20px;
  border: 1px solid var(--border);
  border-radius: 12px;
  background: var(--bg-alt);
}

.exam-summary p { margin: 0 0 12px; }
.exam-summary .bar { margin-bottom: 16px; }
.exam-summary .btn { border: none; font: inherit; font-weight: 600; cursor: pointer; }

.exam-section { margin-top: 40px; }
.exam-section h2 { font-size: 1.35rem; margin: 0; }

.bar {
  display: block;
  height: 6px;
  border-radius: 3px;
  background: var(--border);
  overflow: hidden;
}

.bar span { display: block; height: 100%; background: var(--accent); }

.quiz-list {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 12px;
  margin: 0;
  padding: 0;
  list-style: none;
}

.quiz-row {
  display: flex;
  flex-direction: column;
  gap: 6px;
  height: 100%;
  padding: 14px 16px;
  border: 1px solid var(--border);
  border-radius: 12px;
  background: var(--bg);
  color: var(--text);
  text-decoration: none;
}

.quiz-row:hover { border-color: var(--accent); }
.quiz-num { color: var(--accent); font-size: 0.8rem; font-weight: 600; text-transform: uppercase; letter-spacing: 0.06em; }
.quiz-row strong { line-height: 1.35; }
.quiz-status { color: var(--muted); font-size: 0.9rem; }
.quiz-status.is-done { color: var(--good); }

.tally {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  margin: 16px 0 8px;
  padding: 0;
  list-style: none;
}

.tally-btn {
  width: 34px;
  height: 34px;
  border: 1.5px solid var(--border);
  border-radius: 6px;
  background: var(--bg);
  color: var(--muted);
  font: inherit;
  font-size: 0.85rem;
  cursor: pointer;
}

.tally-btn.is-right { background: var(--good); border-color: var(--good); color: var(--bg); }
.tally-btn.is-wrong { background: var(--bad); border-color: var(--bad); color: var(--bg); }
.tally-btn[aria-current] { outline: 2px solid var(--accent); outline-offset: 2px; }

.tally-key { display: flex; flex-wrap: wrap; align-items: center; gap: 6px; color: var(--muted); font-size: 0.85rem; margin: 0 0 20px; }
.key { display: inline-block; width: 12px; height: 12px; border: 1.5px solid var(--border); border-radius: 3px; margin-left: 8px; }
.key:first-child { margin-left: 0; }
.key.is-right { background: var(--good); border-color: var(--good); }
.key.is-wrong { background: var(--bad); border-color: var(--bad); }

.q-card {
  padding: 20px;
  border: 1px solid var(--border);
  border-radius: 12px;
  background: var(--bg-alt);
}

.q-num { margin: 0 0 8px; color: var(--muted); font-size: 0.9rem; }
.q-text { margin: 0 0 16px; font-size: 1.1rem; font-weight: 600; }
.q-options { display: grid; gap: 8px; }

.q-opt {
  display: flex;
  gap: 12px;
  align-items: flex-start;
  width: 100%;
  padding: 12px 14px;
  border: 1.5px solid var(--border);
  border-radius: 8px;
  background: var(--bg);
  color: var(--text);
  font: inherit;
  text-align: left;
  cursor: pointer;
}

.q-opt:hover:not(:disabled) { border-color: var(--accent); }
.q-opt:disabled { cursor: default; }
.q-opt.is-right { border-color: var(--good); }
.q-opt.is-wrong { border-color: var(--bad); }
.q-letter { flex: none; font-weight: 700; color: var(--muted); }
.q-opt.is-right .q-letter { color: var(--good); }
.q-opt.is-wrong .q-letter { color: var(--bad); }

.q-explain { margin-top: 16px; padding-top: 16px; border-top: 1px solid var(--border); }
.q-explain p { margin: 0 0 8px; }
.q-verdict { font-weight: 700; }
.q-verdict.is-right { color: var(--good); }
.q-verdict.is-wrong { color: var(--bad); }

.q-nav { display: flex; flex-wrap: wrap; gap: 8px; align-items: center; margin: 16px 0; }
.q-nav .btn-small { margin-top: 0; }
.q-nav .btn { padding: 8px 16px; border: none; font: inherit; font-weight: 600; cursor: pointer; }
.btn-small:disabled { opacity: 0.5; cursor: default; }
.q-reset { margin-top: 24px; }