:root {
  --paper: #f6f1e7;
  --ink: #1c1917;
  --muted: #57534e;
  --line: #d6cbb8;
  --navy: #1e3a5f;
  --blue: #2563eb;
  --blue-bg: #e8f0fe;
  --yellow: #b45309;
  --yellow-bg: #fef6d8;
  --red: #b91c1c;
  --red-bg: #fde8e8;
  --purple: #6d28d9;
  --purple-bg: #f3e8ff;
  --green: #166534;
  --green-bg: #e8f6ec;
  --skill: #0f766e;
  --skill-bg: #e6f4f1;
  --parent: #44403c;
  --parent-bg: #f0ece4;
  --side: #fffdf8;
  --text: 20px;
  --side-w: 268px;
  --nav-h: 52px;
}

* { box-sizing: border-box; }

.skip-link {
  position: absolute;
  left: -999px;
  top: 8px;
  z-index: 80;
  padding: 8px 14px;
  background: #1e3a5f;
  color: #fff;
  text-decoration: none;
  border-radius: 6px;
  font-family: "PingFang SC", "Noto Sans SC", sans-serif;
}
.skip-link:focus {
  left: 12px;
}

a:focus-visible,
button:focus-visible,
textarea:focus-visible,
summary:focus-visible {
  outline: 3px solid #2563eb;
  outline-offset: 2px;
}

html {
  font-size: var(--text);
  scroll-behavior: smooth;
  scroll-padding-top: 72px;
}

body {
  margin: 0;
  color: var(--ink);
  background: var(--paper);
  font-family: "Songti SC", "STSong", "Noto Serif SC", "Source Han Serif SC", serif;
  line-height: 1.85;
  display: grid;
  grid-template-columns: var(--side-w) minmax(0, 1fr);
  grid-template-rows: var(--nav-h) 1fr;
  min-height: 100vh;
}

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

.site-nav {
  grid-column: 1 / -1;
  position: sticky;
  top: 0;
  z-index: 30;
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 8px 20px;
  height: var(--nav-h);
  padding: 0 24px;
  border-bottom: 1px solid var(--line);
  background: var(--side);
  font-size: 0.88rem;
  font-family: "PingFang SC", "Noto Sans SC", sans-serif;
}

.site-nav a { text-decoration: none; }
.site-nav a[aria-current="page"] { font-weight: 700; }

.side-toc {
  grid-column: 1;
  grid-row: 2;
  position: sticky;
  top: var(--nav-h);
  align-self: start;
  height: calc(100vh - var(--nav-h));
  overflow-y: auto;
  padding: 18px 16px 48px;
  border-right: 1px solid var(--line);
  background: var(--side);
  font-family: "PingFang SC", "Noto Sans SC", sans-serif;
  font-size: 0.8rem;
  line-height: 1.45;
}

.side-toc .side-title {
  font-size: 0.72rem;
  letter-spacing: 0.08em;
  color: var(--muted);
  margin: 0 0 10px;
}

.side-toc .group {
  margin: 0 0 10px;
}

.side-toc .group-name {
  font-weight: 700;
  color: var(--navy);
  margin: 0;
  padding: 6px 8px;
  cursor: pointer;
  list-style: none;
  border-radius: 6px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  user-select: none;
}

.side-toc .group-name::-webkit-details-marker { display: none; }

.side-toc .group-name::after {
  content: "▾";
  font-size: 0.72rem;
  color: var(--muted);
  transform: rotate(-90deg);
}

.side-toc details[open] > .group-name::after {
  transform: rotate(0deg);
}

.side-toc .group-name:hover { background: #f3eee4; }

.side-toc details ol { margin: 4px 0 8px; padding-left: 6px; }

.side-toc ol {
  list-style: none;
  margin: 0;
  padding: 0;
}

.side-toc li { margin: 0; }

.side-toc a,
.side-toc .soon {
  display: block;
  padding: 5px 8px;
  border-radius: 6px;
  text-decoration: none;
}

.side-toc a:hover { background: #f3eee4; }

.side-toc a[aria-current="page"] {
  background: var(--blue-bg);
  color: var(--blue);
  font-weight: 700;
}

.side-toc a.is-seen:not([aria-current="page"]) {
  box-shadow: inset 3px 0 0 #86efac;
}

.side-toc .soon {
  color: #a8a29e;
}

.wrap {
  grid-column: 2;
  max-width: 860px;
  width: 100%;
  margin: 0 auto;
  padding: 36px 40px 96px;
}

h1, h2, h3, h4 {
  font-family: "PingFang SC", "Hiragino Sans GB", "Noto Sans SC", sans-serif;
  color: var(--navy);
  line-height: 1.4;
}

h1 { font-size: 2rem; margin: 0 0 0.7em; }
h2 { font-size: 1.4rem; margin: 2em 0 0.8em; border-bottom: 2px solid var(--navy); padding-bottom: 0.25em; }
h3 { font-size: 1.16rem; margin: 1.6em 0 0.55em; }
h4 { font-size: 1.02rem; margin: 1.3em 0 0.45em; }

.lead {
  font-size: 1.08rem;
  color: var(--muted);
  margin: 0 0 1.4em;
}

.meta-line {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin: 0.4em 0 1.6em;
  font-family: "PingFang SC", "Noto Sans SC", sans-serif;
  font-size: 0.82rem;
}

.chip {
  border: 1px solid var(--line);
  background: #fff;
  padding: 3px 12px;
  border-radius: 999px;
}

.talk {
  font-family: "Kaiti SC", "STKaiti", "KaiTi", serif;
  font-size: 1.08rem;
}

.box {
  border: 1.5px solid currentColor;
  border-left-width: 8px;
  background: #fff;
  padding: 14px 18px 16px;
  margin: 1.2em 0;
}

.box-label {
  font-family: "PingFang SC", "Noto Sans SC", sans-serif;
  font-size: 0.78rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  margin-bottom: 0.45em;
}

.box-blue { color: var(--blue); background: var(--blue-bg); }
.box-yellow { color: var(--yellow); background: var(--yellow-bg); }
.box-red { color: var(--red); background: var(--red-bg); }
.box-purple { color: var(--purple); background: var(--purple-bg); }
.box-green { color: var(--green); background: var(--green-bg); }
.box-skill { color: var(--skill); background: var(--skill-bg); }
.box-parent { color: var(--parent); background: var(--parent-bg); }

.box p, .box li { color: var(--ink); }
.box .box-label { color: inherit; }
.box p:last-child, .box ul:last-child, .box ol:last-child { margin-bottom: 0; }

.katex { font-size: 1.28em !important; }
.katex-display {
  font-size: 1.5em !important;
  margin: 1.1em 0 !important;
  overflow-x: auto;
  overflow-y: hidden;
}

.figure {
  display: block;
  width: 100%;
  max-width: 680px;
  margin: 1.2em auto;
  background: #fff;
  border: 1px solid var(--line);
  padding: 12px;
}

.figcap {
  text-align: center;
  color: var(--muted);
  font-size: 0.88rem;
  margin: 0.2em 0 1.4em;
}

.example {
  border: 1px solid var(--line);
  background: #fff;
  padding: 18px 20px 8px;
  margin: 1.8em 0;
  border-radius: 8px;
}

.example h3 { margin-top: 0; }

.stem {
  font-size: 1.1rem;
  font-weight: 600;
  line-height: 2.4;
  margin: 0.4em 0 1em;
}

.meta-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 10px 16px;
  margin: 1em 0 1.2em;
  font-size: 0.95rem;
}

.meta-grid div {
  background: var(--paper);
  border: 1px solid var(--line);
  padding: 10px 12px;
}

.work,
.work-sm {
  display: block;
  margin: 0.35em 0 0.5em;
}
.work-input {
  display: block;
  width: 100%;
  min-height: 7.2em;
  padding: 8px 12px;
  border: 1px dashed #b7ad9a;
  border-radius: 6px;
  background: repeating-linear-gradient(
    to bottom,
    #fff 0,
    #fff 31px,
    #eadfcb 32px
  );
  background-attachment: local;
  font-family: inherit;
  font-size: 1rem;
  line-height: 32px;
  color: var(--ink);
  resize: vertical;
}
.work-sm .work-input { min-height: 3.4em; }
.work-input:focus {
  border-color: #2563eb;
  background-color: #fff;
}
.q-kind {
  display: inline-block;
  margin-right: 0.45em;
  font-size: 0.72rem;
  font-weight: 700;
  padding: 0.08em 0.5em;
  border-radius: 4px;
  vertical-align: middle;
  white-space: nowrap;
  letter-spacing: 0.02em;
}
.q-kind-app { background: #e8f5e9; color: #1b7a32; }
.q-kind-geo { background: #e3f2fd; color: #1565c0; }
.q-kind-calc { background: #fff3e0; color: #e65100; }
.q-kind-eq { background: #f3e5f5; color: #6a1b9a; }
.q-kind-simp { background: #e0f7fa; color: #00695c; }
.q-kind-fill { background: #fce4ec; color: #c2185b; }
.q-kind-choice { background: #e8eaf6; color: #283593; }
.q-kind-oral { background: #efebe9; color: #5d4037; }
.q-kind-eval { background: #fff8e1; color: #f9a825; }
.q-kind-sys { background: #ede7f6; color: #4527a0; }
.q-kind-ineq { background: #fce4ec; color: #ad1457; }
.q-kind-coord { background: #e0f2f1; color: #00695c; }
.q-kind-stat { background: #fff3e0; color: #ef6c00; }
.q-kind-other { background: #eceff1; color: #455a64; }
.q-src {
  display: inline-block;
  margin-left: 0.45em;
  font-size: 0.72rem;
  font-weight: 700;
  padding: 0.08em 0.5em;
  border-radius: 4px;
  vertical-align: middle;
  white-space: nowrap;
}
.q-src-gd { background: #bbdefb; color: #0d47a1; }
.q-src-hn { background: #ffccbc; color: #bf360c; }
.q-src-other { background: #f3ebe0; color: #6b5e4f; }
.exam-banner {
  background: #1e3a5f;
  color: #fff;
  padding: 0.75em 1.1em;
  border-radius: 6px;
  font-weight: 600;
  margin: 0.6em 0 1.4em;
}
.choices {
  list-style: none;
  padding-left: 0.1em;
  margin: 0.25em 0 0.35em;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0.1em 1.4em;
}
.choices li {
  margin: 0.12em 0;
  line-height: 1.85;
}

ol, ul { padding-left: 1.4em; }

ol li, ul li {
  margin: 0.7em 0;
  line-height: 2.5;
}

.practice { margin: 0.6em 0 1.6em; }
.practice li { margin: 1em 0 0.4em; line-height: 2.6; }
.q-fig .figure {
  max-width: 520px;
  margin: 0.6em 0 0.8em;
}
.q-fig { margin: 0.4em 0 0.2em; }

.toc { padding-left: 1.2em; }
.toc a { text-decoration: none; }
.toc li { margin: 0.45em 0; line-height: 1.7; }

.soon { color: #a8a29e; }

.wrong-list { list-style: none; padding: 0; }
.wrong-list li {
  background: #fff;
  border: 1px solid var(--line);
  border-radius: 8px;
  padding: 12px 16px;
  margin: 0 0 12px;
  line-height: 1.6;
}
.wrong-list .wrong-meta {
  font-size: 0.82rem;
  color: var(--muted);
  font-family: "PingFang SC", "Noto Sans SC", sans-serif;
  margin: 0 0 6px;
}
.wrong-empty { color: var(--muted); }

.answer-item {
  break-inside: avoid;
  margin: 0 0 1.4em;
  padding: 0.2em 0 0.8em;
}
.answer-item + .answer-item {
  border-top: 1px dashed var(--line);
  padding-top: 1em;
}
.answers-block .box {
  margin-top: 1.8em;
}
.back-list {
  margin: 0.2em 0 0;
  padding-left: 1.3em;
}
.back-list li {
  margin: 0.35em 0;
  line-height: 1.7;
}
.answer-item h3 {
  border-left: 6px solid var(--green);
  padding-left: 10px;
  margin-top: 0.4em;
}

.steps { padding-left: 1.3em; }
.steps li { margin: 0.45em 0; line-height: 2.2; }

.practice-block { margin-top: 2.4em; }
.q-actions {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4em 0.7em;
  margin: 0.15em 0 1.1em;
}
.ans-btn,
.wrong-btn,
.exam-unlock,
.gate-done,
.gate-reset {
  display: inline-block;
  padding: 0.28em 0.85em;
  font-size: 0.82rem;
  font-family: "PingFang SC", "Noto Sans SC", sans-serif;
  font-weight: 600;
  color: #1e3a5f;
  background: #e8f0fe;
  border: 1px solid #c5d4f0;
  border-radius: 6px;
  cursor: pointer;
}
.ans-btn:hover, .exam-unlock:hover, .gate-done:hover { background: #d7e4fb; }
.ans-btn.is-open { background: #efebe9; color: #5d4037; }
.wrong-btn {
  color: #9a3412;
  background: #ffedd5;
  border-color: #fdba74;
}
.wrong-btn:hover { background: #fed7aa; }
.wrong-btn:disabled, .exam-unlock:disabled, .gate-done:disabled {
  opacity: 0.55;
  cursor: default;
}
.gate-reset {
  color: #57534e;
  background: #f5f5f4;
  border-color: #d6d3d1;
}
.exam-toolbar {
  background: #fff;
  border: 1px solid var(--line);
  border-radius: 8px;
  padding: 0.8em 1em;
  margin: 0 0 1.2em;
}
.exam-toolbar p { margin: 0 0 0.6em; }
body[data-exam="1"]:not(.exam-open) .q-actions { display: none; }
.gate-actions { margin: 0.2em 0 1.2em; }
.q-answer {
  margin: 0.35em 0 0.8em;
  padding: 0.6em 0.9em 0.2em;
  background: #fff;
  border: 1px solid var(--line);
  border-radius: 8px;
}
.q-answer .box { margin: 0.6em 0; }
.gate-bar {
  display: flex;
  flex-wrap: wrap;
  gap: 0.45em;
  margin: 0.4em 0 1.2em;
}
.gate-chip {
  font-size: 0.78rem;
  font-family: "PingFang SC", "Noto Sans SC", sans-serif;
  font-weight: 700;
  padding: 0.2em 0.7em;
  border-radius: 999px;
  background: #eceff1;
  color: #78909c;
}
.gate-chip.is-on { background: #e8f5e9; color: #1b7a32; }
.gate-lock {
  opacity: 0.42;
  pointer-events: none;
  filter: grayscale(0.4);
}
.gate-lock .ans-btn { display: none; }
.side-toc a.locked { color: #b0a99f; }

.answers-block {
  margin-top: 2.8em;
  padding-top: 0.4em;
}

footer.page-end {
  margin-top: 3.2em;
  padding-top: 1.1em;
  border-top: 1px solid var(--line);
  color: var(--muted);
  font-size: 0.85rem;
  font-family: "PingFang SC", "Noto Sans SC", sans-serif;
}

@media print {
  :root { --paper: #fff; }
  body {
    display: block;
    background: #fff;
    font-size: 18px;
  }
  .site-nav, .side-toc, .no-print { display: none !important; }
  .wrap { max-width: none; padding: 0; }
  .box, .example, .answer-item, .work { break-inside: avoid; }
  .work-input { min-height: 8.5em; }
  .work-sm .work-input { min-height: 2.6em; }
  a { color: inherit; text-decoration: none; }
  .katex { font-size: 1.35em !important; }
  .katex-display { font-size: 1.6em !important; }
}

@media (max-width: 900px) {
  body {
    display: block;
  }
  .site-nav {
    position: sticky;
    height: auto;
    min-height: var(--nav-h);
    padding: 10px 16px;
  }
  .side-toc {
    position: static;
    height: auto;
    max-height: 42vh;
    border-right: none;
    border-bottom: 1px solid var(--line);
  }
  .wrap { padding: 24px 18px 72px; }
  .meta-grid { grid-template-columns: 1fr; }
  html { font-size: 18px; }
}