:root {
  --bg: #f6f8f7;
  --surface: #ffffff;
  --surface-soft: #eef7f5;
  --ink: #17202a;
  --muted: #5b6773;
  --line: #d9e3e0;
  --teal: #0d9488;
  --teal-deep: #0f766e;
  --orange: #ea580c;
  --blue: #2563eb;
  --shadow: 0 16px 40px rgba(23, 32, 42, 0.08);
  --radius: 8px;
  --max: 1240px;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body { margin: 0; background: var(--bg); color: var(--ink); font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", sans-serif; font-size: 16px; line-height: 1.68; }
a { color: inherit; text-decoration: none; }
code { font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", monospace; }
.skip-link { position: absolute; left: 16px; top: -80px; z-index: 60; padding: 10px 14px; background: var(--ink); color: #fff; border-radius: var(--radius); }
.skip-link:focus { top: 16px; outline: 3px solid rgba(13, 148, 136, 0.32); }
.site-header { position: sticky; top: 0; z-index: 40; border-bottom: 1px solid var(--line); background: rgba(255,255,255,0.95); backdrop-filter: blur(12px); }
.header-inner { width: min(var(--max), calc(100% - 32px)); margin: 0 auto; min-height: 66px; display: flex; align-items: center; justify-content: space-between; gap: 18px; }
.brand { display: flex; align-items: center; gap: 12px; min-width: 0; }
.brand-mark { width: 40px; height: 40px; border-radius: var(--radius); background: var(--teal); color: #fff; display: grid; place-items: center; flex: 0 0 auto; }
.brand strong { display: block; font-size: 18px; line-height: 1.2; }
.brand small { display: block; color: var(--muted); line-height: 1.35; }
.top-nav { display: flex; align-items: center; justify-content: flex-end; gap: 8px; flex-wrap: wrap; }
.top-nav a, .button, .route-card a, .pager a, .toc-list a, .progress-list a { border: 1px solid var(--line); background: #fff; border-radius: var(--radius); min-height: 44px; display: inline-flex; align-items: center; justify-content: center; padding: 9px 12px; color: var(--ink); transition: transform 160ms ease, border-color 160ms ease, background 160ms ease; }
.top-nav a:hover, .button:hover, .route-card a:hover, .pager a:hover, .toc-list a:hover, .progress-list a:hover, .chapter-card:hover { transform: translateY(-1px); border-color: rgba(13, 148, 136, 0.45); }
.top-nav a:focus-visible, .button:focus-visible, .route-card a:focus-visible, .pager a:focus-visible, .toc-list a:focus-visible, .progress-list a:focus-visible { outline: 3px solid rgba(13, 148, 136, 0.28); outline-offset: 2px; }
.button.primary { background: var(--teal); border-color: var(--teal); color: #fff; font-weight: 800; }
.hero { width: min(var(--max), calc(100% - 32px)); margin: 0 auto; padding: 42px 0 34px; display: grid; grid-template-columns: minmax(0, 1.1fr) minmax(320px, 0.9fr); gap: 28px; align-items: center; }
.hero-copy { display: grid; gap: 18px; }
.eyebrow { display: inline-flex; width: fit-content; align-items: center; padding: 6px 10px; border: 1px solid rgba(13, 148, 136, 0.2); color: var(--teal-deep); background: var(--surface-soft); border-radius: var(--radius); font-weight: 800; font-size: 13px; }
.hero h1, .chapter-hero h1 { margin: 0; font-size: clamp(34px, 5.5vw, 64px); line-height: 1.08; letter-spacing: 0; }
.hero p, .chapter-hero p { margin: 0; color: var(--muted); font-size: 18px; max-width: 780px; }
.hero-actions { display: flex; gap: 10px; flex-wrap: wrap; }
.hero-panel { border: 1px solid var(--line); border-radius: var(--radius); background: #fff; box-shadow: var(--shadow); padding: 18px; }
.hero-panel svg { width: 100%; height: auto; display: block; }
.overview-band { border-top: 1px solid var(--line); border-bottom: 1px solid var(--line); background: #fff; }
.overview-grid { width: min(var(--max), calc(100% - 32px)); margin: 0 auto; padding: 18px 0; display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 12px; }
.overview-grid article { border: 1px solid var(--line); border-radius: var(--radius); padding: 16px; min-height: 92px; background: #fbfdfc; }
.overview-grid strong { display: block; color: var(--teal-deep); font-size: 28px; line-height: 1; }
.overview-grid span { display: block; color: var(--muted); margin-top: 8px; }
.content-section { width: min(var(--max), calc(100% - 32px)); margin: 0 auto; padding: 34px 0; }
.section-heading { display: grid; gap: 10px; margin-bottom: 16px; }
.section-heading h2 { margin: 0; font-size: clamp(26px, 3vw, 38px); line-height: 1.18; }
.route-grid, .chapter-grid { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 16px; }
.route-card, .chapter-card, .reading-card, .lesson-block, .chapter-aside, .formula-grid div { border: 1px solid var(--line); background: var(--surface); border-radius: var(--radius); }
.route-card { padding: 18px; display: grid; gap: 12px; align-content: start; }
.route-card h3, .chapter-card h3, .lesson-block h2, .reading-card h2, .chapter-aside h2 { margin: 0; line-height: 1.24; }
.route-card p, .chapter-card p { margin: 0; color: var(--muted); }
.route-card a { width: fit-content; color: var(--teal-deep); font-weight: 800; }
.formula-grid { display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 12px; }
.formula-grid div { padding: 14px; display: grid; gap: 8px; }
.formula-grid code { color: #0f3f3a; background: #f1f5f4; border: 1px solid var(--line); border-radius: 6px; padding: 4px 6px; white-space: normal; }
.formula-grid span { color: var(--muted); font-size: 14px; }
.chapter-card { padding: 18px; display: grid; gap: 12px; transition: transform 160ms ease, border-color 160ms ease; }
.chapter-card .part { color: var(--teal-deep); font-size: 13px; font-weight: 800; }
.chapter-card .meta, .chapter-meta { display: flex; flex-wrap: wrap; gap: 8px; }
.tag { display: inline-flex; align-items: center; min-height: 28px; border-radius: 999px; background: #f1f5f4; color: #40505c; padding: 3px 8px; font-size: 13px; font-weight: 700; }
.chapter-shell { width: min(var(--max), calc(100% - 32px)); margin: 0 auto; padding: 28px 0 56px; display: grid; grid-template-columns: 280px minmax(0, 1fr); gap: 24px; align-items: start; }
.chapter-aside { position: sticky; top: 90px; padding: 16px; display: grid; gap: 16px; }
.progress-list, .toc-list { display: grid; gap: 7px; }
.progress-list a, .toc-list a { justify-content: flex-start; color: var(--muted); }
.progress-list a.current, .toc-list a:hover { color: var(--teal-deep); background: var(--surface-soft); border-color: rgba(13, 148, 136, 0.22); font-weight: 800; }
.chapter-main { display: grid; gap: 18px; min-width: 0; }
.chapter-hero { border-bottom: 1px solid var(--line); background: #fff; }
.chapter-hero-inner { width: min(var(--max), calc(100% - 32px)); margin: 0 auto; padding: 34px 0; display: grid; gap: 14px; }
.reading-card, .lesson-block { padding: 20px; }
.reading-card ul, .lesson-block ul, .chapter-aside ul { margin: 10px 0 0; padding-left: 20px; }
.reading-card li, .lesson-block li, .chapter-aside li { margin: 8px 0; }
.lesson-block { display: grid; gap: 14px; }
.lesson-section { border-top: 1px solid var(--line); padding-top: 14px; }
.lesson-section:first-of-type { border-top: 0; padding-top: 0; }
.lesson-section h3 { margin: 0 0 8px; font-size: 20px; line-height: 1.28; }
.lesson-section p { margin: 8px 0; }
.concept-grid { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 12px; }
.concept { border: 1px solid var(--line); border-radius: var(--radius); padding: 14px; background: #fbfdfc; }
.concept strong { display: block; margin-bottom: 6px; color: var(--teal-deep); }
.example-box { border-left: 4px solid var(--orange); background: #fff7ed; border-radius: 0 var(--radius) var(--radius) 0; padding: 14px 16px; margin-top: 12px; }
.formula-box { border-left: 4px solid var(--blue); background: #eff6ff; border-radius: 0 var(--radius) var(--radius) 0; padding: 14px 16px; margin-top: 12px; }
.formula-box p, .example-box p { margin: 8px 0 0; }
.pager { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 12px; }
.pager a { min-height: 58px; justify-content: flex-start; }
.pager .next { justify-content: flex-end; text-align: right; }
.site-footer { width: min(var(--max), calc(100% - 32px)); margin: 0 auto; padding: 20px 0 36px; color: var(--muted); font-size: 14px; }
@media (max-width: 1080px) { .hero, .chapter-shell { grid-template-columns: 1fr; } .chapter-aside { position: static; } .overview-grid, .formula-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); } .route-grid, .chapter-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); } }
@media (max-width: 720px) { .header-inner { align-items: stretch; flex-direction: column; padding: 12px 0; } .top-nav { justify-content: stretch; } .top-nav a, .button { flex: 1 1 auto; } .hero { padding-top: 28px; } .overview-grid, .formula-grid, .route-grid, .chapter-grid, .concept-grid, .pager { grid-template-columns: 1fr; } }
@media (prefers-reduced-motion: reduce) { *, *::before, *::after { scroll-behavior: auto !important; transition: none !important; } }