:root {
  --paper: #fafafa;
  --ink: #262629;
  --muted: #6f6f74;
  --accent: #45454b;
  --number: #86868b;
  --line: #e2e2e5;
  --soft: #f0f0f2;
  --quote: #f4f4f6;
  --panel: #fff;
  --panel-shadow: #18181b0d;
  --nav-height: 56px;
  color-scheme: light;
  scrollbar-color: #b5b5ba var(--paper);
}
html.dark {
  --paper: #19191b;
  --ink: #e8e8ec;
  --muted: #a1a1a8;
  --accent: #cdcdd4;
  --number: #9999a0;
  --line: #37373d;
  --soft: #27272b;
  --quote: #222225;
  --panel: #1f1f22;
  --panel-shadow: #00000030;
  color-scheme: dark;
  scrollbar-color: #5f5f67 var(--paper);
}
*, *::before, *::after { box-sizing: border-box; }
html { background: var(--paper); scroll-behavior: auto; }
body { margin: 0; color: var(--ink); background: var(--paper); font: 16px/1.9 -apple-system, BlinkMacSystemFont, "PingFang SC", "Microsoft YaHei", sans-serif; -webkit-font-smoothing: antialiased; }
::selection { color: var(--ink); background: var(--line); }
a { color: inherit; text-decoration: none; }
button, summary, a { -webkit-tap-highlight-color: transparent; }
a:hover { color: var(--accent); }
:is(a,button,summary):focus-visible { outline: 2px solid var(--accent); outline-offset: 5px; border-radius: 2px; }
:is(main,.lesson,.chapter):focus { outline: none; }
.page-width { width: min(var(--lm-canvas-content-width), calc(100% - 2 * var(--lm-page-gutter))); margin-inline: auto; }
.skip-link { position: fixed; z-index: 200; left: 20px; top: -100px; padding: 10px 18px; background: var(--panel); }
.skip-link:focus { top: 12px; }
.hero { padding-block: 46px 32px; display: grid; grid-template-columns: 1fr 242px; gap: 48px; align-items: end; }
.eyebrow { margin: 0 0 16px; color: var(--accent); font-size: 11px; font-weight: 600; letter-spacing: .12em; }
h1 { margin: 0; font-family: "Songti SC", "Noto Serif CJK SC", "STSong", serif; font-size: clamp(36px, 4.1vw, 52px); line-height: 1.3; font-weight: 600; letter-spacing: -.025em; }
.hero-description { margin: 18px 0 0; font-size: 15px; letter-spacing: .035em; }
.hero-index { display: grid; margin-bottom: 6px; }
.hero-index a { display: grid; grid-template-columns: 58px 1fr 16px; align-items: center; gap: 10px; padding-block: 15px; border-top: 1px solid var(--line); }
.hero-index a:last-child { border-bottom: 1px solid var(--line); }
.hero-index a > span { color: var(--muted); font: 11px/1.5 "SFMono-Regular", Consolas, monospace; }
.hero-index strong { font-size: 15px; font-weight: 500; }
.hero-index a:hover strong { color: var(--accent); }
.chapter-bar { position: sticky; top: var(--lm-chrome-offset); z-index: 70; display: flex; align-items: center; min-height: var(--nav-height); border-block: 1px solid var(--line); background: var(--paper); }
.group-nav { display: flex; align-items: stretch; align-self: stretch; gap: 28px; }
.group-nav a { display: flex; align-items: center; min-height: 44px; font-size: 13px; color: var(--muted); border-bottom: 2px solid transparent; margin-bottom: -1px; }
.group-nav a[aria-current] { color: var(--ink); border-color: var(--accent); font-weight: 600; }
.group-nav a:hover { color: var(--ink); }
.reading-position { margin-left: auto; padding-left: 20px; font: 12px/1.5 "SFMono-Regular", Consolas, monospace; font-variant-numeric: tabular-nums; color: var(--accent); white-space: nowrap; }
.reading-position span { display: inline-block; margin-left: 12px; font-family: -apple-system, "PingFang SC", sans-serif; font-size: 13px; font-weight: 500; color: var(--ink); min-width: 6em; }
.contents { margin-left: 20px; flex-shrink: 0; }
.contents summary { min-width: 44px; min-height: 44px; padding: 8px 0 8px 14px; display: flex; align-items: center; gap: 12px; color: var(--ink); cursor: pointer; font-size: 13px; list-style: none; }
.contents summary::-webkit-details-marker { display: none; }
.contents-glyph { font-size: 20px; line-height: 1; font-weight: 300; }
.contents[open] .contents-glyph { transform: rotate(45deg); }
.contents summary:hover { color: var(--accent); }
/* An open directory is part of the chrome, including its overhanging panel. */
:root.reading-focus.reader-autohide:has(.contents[open], .chapter-bar :focus-visible) {
  --lm-chrome-offset: var(--header-height);
}
:root.reading-focus.reader-autohide:has(.contents[open], .chapter-bar :focus-visible) .reading-shell-header {
  opacity: 1;
  pointer-events: auto;
}
.toc-panel { position: absolute; top: calc(100% + 1px); left: 0; right: 0; padding: 16px 22px; background: var(--panel); border: 1px solid var(--line); box-shadow: 0 14px 22px var(--panel-shadow); max-height: var(--toc-max-height, min(580px, 60dvh)); overflow: auto; overscroll-behavior: contain; scrollbar-gutter: stable; scrollbar-width: thin; }
.toc-inner { display: grid; grid-template-columns: 1fr 1fr 2fr; gap: 24px; }
.toc-group > p { display: flex; justify-content: space-between; color: var(--accent); font-size: 12px; font-weight: 600; margin: 0 0 6px; border-bottom: 1px solid var(--line); padding-bottom: 8px; }
.toc-group > p span { font: 10px/1.9 "SFMono-Regular", Consolas, monospace; color: var(--muted); }
.toc-group ol { list-style: none; padding: 0; margin: 0; }
.toc-group:last-child ol { display: grid; grid-template-columns: 1fr 1fr; column-gap: 20px; }
.toc-group a { display: flex; align-items: center; min-height: 44px; gap: 12px; font-size: 13px; }
.toc-group a:hover { color: var(--ink); }
.toc-group a span { font: 10px/1.5 "SFMono-Regular", Consolas, monospace; color: var(--muted); }
.toc-group a[aria-current] { color: var(--accent); font-weight: 650; }
.demo-page .chapter, .reading-focus .demo-page .chapter { scroll-margin-top: calc(var(--lm-chrome-offset) + var(--nav-height) + 26px); }
.chapters { padding-bottom: 36px; }
.chapter-heading { padding-top: 36px; margin-bottom: 4px; }
.chapter-heading > p { color: var(--accent); margin: 0 0 6px; font-size: 11px; letter-spacing: .08em; }
.chapter-heading > p span { margin-left: 16px; color: var(--muted); font-family: "SFMono-Regular", Consolas, monospace; }
.chapter-heading h2 { font-family: "Songti SC", "STSong", serif; font-size: 32px; font-weight: 500; line-height: 1.5; margin: 0; }
.chapter + .chapter { border-top: 1px solid var(--line); margin-top: 36px; }
.lesson { display: grid; grid-template-columns: 188px minmax(0, 1fr); column-gap: 48px; padding-block: 52px 62px; scroll-margin-top: calc(var(--lm-chrome-offset) + var(--nav-height) + 24px); }
.chapter-heading + .lesson { padding-top: 30px; }
.lesson + .lesson { border-top: 1px solid var(--line); }
.lesson-heading { align-self: start; position: sticky; top: calc(var(--lm-chrome-offset) + var(--nav-height) + 32px); }
.lesson-number { color: var(--number); font: 300 54px/1 "Helvetica Neue", Arial, sans-serif; letter-spacing: -.05em; }
.lesson-heading h3 { margin: 20px 0 10px; font-size: 25px; line-height: 1.5; font-weight: 600; letter-spacing: -.02em; }
.lesson-heading > p { margin: 0; color: var(--muted); font-size: 14px; line-height: 1.85; text-wrap: pretty; }
.lesson-content { min-width: 0; font-size: 16px; line-height: 1.95; }
.lesson-content p { margin: 0 0 14px; text-wrap: pretty; }
.lesson-content .rubric { font-size: 13px; color: var(--accent); font-weight: 600; margin: 32px 0 14px; line-height: 1.8; letter-spacing: .025em; }
.lesson-content > .rubric:first-child { margin-top: 0; }
.formula { background: var(--soft); padding: 20px 24px; border-radius: 6px; margin: 26px 0 36px; }
.formula h4 { color: var(--accent); font-size: 11px; font-weight: 500; margin: 0 0 8px; }
.formula p { font-size: 18px; line-height: 1.8; font-weight: 550; margin: 0; }
.unit { min-width: 0; }
.unit h5 { font-size: 17px; line-height: 1.7; font-weight: 600; margin: 0 0 10px; }
.unit p:last-child { margin-bottom: 0; }
.steps { display: grid; gap: 28px; }
.steps > .unit { display: grid; grid-template-columns: 26px minmax(0, 1fr); gap: 14px; }
.step-number { color: var(--accent); font: 12px/2.5 "SFMono-Regular", Consolas, monospace; }
.unit ul { margin: 12px 0; padding-left: 1.3em; }
.unit li { padding-left: 3px; margin: 6px 0; }
.unit li::marker { color: var(--accent); font-size: .8em; }
.lesson-content blockquote { margin: 14px 0 0; padding: 14px 18px; background: var(--quote); font-size: 15px; color: var(--ink); line-height: 1.85; border-radius: 4px; }
.lesson-content blockquote p { margin: 0; }
.grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0 30px; }
.grid > .unit { padding: 20px 0; border-top: 1px solid var(--line); }
.grid .unit h5 { color: var(--accent); font-size: 14px; }
.grid .unit p { font-size: 14px; margin-bottom: 8px; }
.grid .unit strong { font-size: 16px; font-weight: 550; }
.grid ul { font-size: 14px; }
.grid blockquote { font-size: 15px; padding: 12px 14px; }
.grid blockquote p { font-size: inherit; }
.compare,.pair { display: grid; grid-template-columns: 1fr 1fr; gap: 24px; margin: 16px 0 0; background: var(--quote); padding: 16px 20px; }
.compare > div + div,.pair > .unit + .unit { border-left: 1px solid var(--line); padding-left: 24px; }
.compare h6,.pair h5 { margin: 0 0 8px; color: var(--muted); font-size: 12px; font-weight: 600; line-height: 1.8; }
.compare > div:last-child h6,.pair > .unit:last-child h5 { color: var(--accent); }
.compare p,.pair p { margin: 0; font-size: 15px; line-height: 1.9; }
.notes { display: grid; grid-template-columns: repeat(auto-fit,minmax(150px,1fr)); gap: 24px; border-top: 1px solid var(--line); padding-top: 18px; }
.notes h5 { font-size: 14px; margin-bottom: 6px; color: var(--accent); }
.notes p { font-size: 13px; color: var(--muted); line-height: 1.85; }
/* Each section owns only its leading rule, so neighboring blocks never double it. */
.takeaway { padding-top: 24px; margin-top: 28px; border-top: 1px solid var(--line); }
.takeaway p { color: var(--ink); font-size: 17px; font-weight: 600; line-height: 1.9; margin: 0; text-wrap: pretty; }
@media (min-width: 1200px) {
  .reading-focus .lesson { grid-template-columns: 220px minmax(0, 1fr); column-gap: 80px; }
  .reading-focus .lesson-content { max-width: 820px; }
}
@media (max-width: 760px) {
  .hero { grid-template-columns: 1fr; gap: 20px; padding-block: 32px 24px; }
  h1 { font-size: clamp(34px,6vw,43px); }
  .eyebrow { font-size: 10px; margin-bottom: 14px; }
  .hero-index { grid-template-columns: repeat(3, 1fr); gap: 18px; }
  .hero-index a { grid-template-columns: 1fr 14px; gap: 5px; padding-block: 13px; border-bottom: 1px solid var(--line); }
  .hero-index a > span:first-child { grid-column: 1/-1; font-size: 10px; }
  .hero-index strong { font-size: 13px; }
  .chapter-bar { --nav-height: 52px; }
  :root { --nav-height: 52px; }
  .group-nav { gap: 16px; flex-shrink: 0; }
  .group-nav a { font-size: 12px; }
  .reading-position { padding-left: 12px; font-size: 11px; }
  .reading-position span { display: none; }
  .contents { margin-left: 12px; }
  .contents summary { font-size: 12px; gap: 6px; padding-left: 0; }
  .toc-panel { padding: 16px 20px; }
  .toc-inner { grid-template-columns: 1fr; gap: 18px; }
  .toc-group ol { display: grid; grid-template-columns: 1fr 1fr; column-gap: 16px; }
  .toc-group a { font-size: 13px; }
  .toc-group > p { margin-bottom: 4px; }
  .lesson { grid-template-columns: 1fr; gap: 26px; padding-block: 32px 44px; }
  .lesson-heading { position: static; display: grid; grid-template-columns: 50px 1fr; column-gap: 18px; }
  .lesson-number { grid-row: 1/3; font-size: 43px; padding-top: 4px; }
  .lesson-heading h3 { font-size: 24px; margin: 0 0 4px; }
  .lesson-heading > p { font-size: 13px; }
  .chapter-heading { padding-top: 28px; margin-bottom: 0; }
  .chapter-heading + .lesson { padding-top: 24px; }
  .chapter-heading h2 { font-size: 27px; }
  .chapter + .chapter { margin-top: 20px; }
  .lesson-content { font-size: 15px; line-height: 1.95; }
  .formula { padding: 15px 18px; margin-block: 22px 28px; }
  .formula p { font-size: 16px; }
  .unit h5 { font-size: 16px; }
  .steps > .unit { grid-template-columns: 22px minmax(0, 1fr); gap: 10px; }
  .step-number { font-size: 10px; line-height: 2.8; }
  .lesson-content blockquote { padding: 12px 14px; font-size: 15px; }
  .grid { column-gap: 20px; }
  .grid .unit strong { font-size: 15px; }
  .grid .unit h5 { font-size: 12px; overflow-wrap: anywhere; }
  .compare,.pair { gap: 14px; padding: 14px; }
  .compare > div + div,.pair > .unit + .unit { padding-left: 14px; }
  .compare p,.pair p { font-size: 15px; }
  .notes { grid-template-columns: 1fr; gap: 16px; }
  .takeaway { padding-top: 20px; margin-top: 24px; }
  .takeaway p { font-size: 16px; }
}
@media (max-width: 430px) {
  .group-nav { gap: 12px; }
  .grid { grid-template-columns: 1fr; }
  .compare,.pair { grid-template-columns: 1fr; gap: 14px; }
  .compare > div + div,.pair > .unit + .unit { border-left: 0; border-top: 1px solid var(--line); padding: 12px 0 0; }
}
@media (max-width: 350px) {
  .group-nav { gap: 10px; }
  .reading-position { padding-left: 8px; }
  .contents { margin-left: 8px; }
}
@media (prefers-reduced-motion: reduce) { html { scroll-behavior: auto; } }
@media (forced-colors: active) {
  .formula,.chapter-bar,.toc-panel,.compare,.pair,blockquote { border: 1px solid CanvasText; }
  .group-nav a[aria-current] { border-bottom-color: LinkText; }
}
@media print {
  :root,html.dark { --paper: #fff; --ink: #111; --muted: #444; --accent: #333; --number: #555; --line: #aaa; --soft: #f1f1f1; --quote: #f5f5f5; }
  html,body { background: #fff; color: #111; }
  .reading-shell-header,.reader-floating-controls,.chapter-bar,.skip-link,.hero-index { display: none !important; }
  .page-width { width: 100%; max-width: none; }
  .hero { display: block; padding: 0 0 24px; }
  h1 { font-size: 30px; }
  .lesson { display: block; padding: 24px 0; break-before: auto; }
  .lesson-heading { position: static; display: block; margin-bottom: 20px; }
  .lesson-number { font-size: 22px; }
  .lesson-heading h3 { margin: 4px 0; }
  .lesson-content { font-size: 11pt; max-width: none; }
  .unit,.formula,.takeaway,blockquote { break-inside: avoid; }
  h2,h3,h4,h5,h6 { break-after: avoid; }
  .chapter-heading { padding-top: 24px; }
  .chapters { padding-bottom: 0; }
}