:root {
  --bg: #f6f7f9; --panel: #ffffff; --ink: #1d2430; --muted: #5f6775; --line: #e2e5ea;
  --accent: #2563eb; --tag: #eef2ff; --tag-ink: #3730a3;
  --radius: 10px; --font: "Noto Sans TC", "Microsoft JhengHei", "PingFang TC", system-ui, sans-serif;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0f1218; --panel: #171c25; --ink: #e6e9ef; --muted: #9aa2b1; --line: #2a3140;
    --accent: #60a5fa; --tag: #1e2a44; --tag-ink: #c7d2fe;
  }
}
* { box-sizing: border-box; }
html, body { margin: 0; background: var(--bg); color: var(--ink); font-family: var(--font); font-size: 17px; line-height: 1.7; }
a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }
.top { display: flex; align-items: center; gap: 16px; flex-wrap: wrap; padding: 10px 16px; border-bottom: 1px solid var(--line); background: var(--panel); }
.brand { font-weight: 700; font-size: 18px; color: var(--ink); }
.top nav { display: flex; gap: 14px; flex-wrap: wrap; font-size: 15px; }
.top nav a { color: var(--muted); }
main { max-width: 820px; margin: 0 auto; padding: 16px 16px 40px; }
footer { max-width: 820px; margin: 0 auto; padding: 0 16px 32px; color: var(--muted); font-size: 13px; }
h1 { font-size: 26px; line-height: 1.35; margin: 8px 0 6px; }
.meta { color: var(--muted); font-size: 14px; margin: 0 0 16px; }
.crumb { font-size: 14px; color: var(--muted); margin: 4px 0 0; }
.card { background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius); padding: 16px; margin-bottom: 16px; overflow-wrap: anywhere; }
.card h2 { margin: 0 0 10px; font-size: 19px; }
.card h3 { margin: 16px 0 6px; font-size: 16px; }
.card p { margin: 8px 0; }
.figure { margin: 8px 0 12px; text-align: center; }
.figure svg { max-width: 100%; height: auto; font-family: var(--font); border-radius: 6px; }
.katex-display { overflow-x: auto; overflow-y: hidden; padding: 2px 0; }
.katex { font-size: 1.08em; }
.stem { font-size: 18px; }
ol.choices { padding-left: 0; list-style: none; margin: 8px 0; }
ol.choices li { margin: 4px 0; }
.answer .katex { font-size: 1.25em; }
.answer { margin: 12px 0 0; padding: 10px 12px; border-left: 4px solid var(--accent); background: var(--bg); border-radius: 6px; }
.solution { margin-top: 12px; }
details summary { cursor: pointer; color: var(--accent); font-weight: 600; }
ul.apps { list-style: none; padding: 0; margin: 0; }
ul.apps li { display: flex; gap: 10px; align-items: baseline; padding: 6px 0; border-bottom: 1px solid var(--line); }
ul.apps li:last-child { border-bottom: 0; }
.tag { flex: 0 0 auto; font-size: 13px; padding: 1px 8px; border-radius: 999px; background: var(--tag); color: var(--tag-ink); }
img.illus { display: block; width: 100%; max-width: 640px; height: auto; margin: 8px auto; border-radius: 8px; }
.illus-row { display: grid; gap: 12px; }
@media (min-width: 700px) { .illus-row.two { grid-template-columns: 1fr 1fr; } }
.video { position: relative; width: 100%; aspect-ratio: 16 / 9; border-radius: 8px; overflow: hidden; background: #000; }
.video iframe { position: absolute; inset: 0; width: 100%; height: 100%; border: 0; }
ul.links { padding-left: 20px; margin: 6px 0; }
.stages { display: grid; gap: 12px; grid-template-columns: 1fr; }
@media (min-width: 600px) { .stages { grid-template-columns: 1fr 1fr; } }
.stages a { display: block; background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius); padding: 16px; color: var(--ink); }
.stages a b { font-size: 20px; color: var(--accent); }
.stages a span { display: block; color: var(--muted); font-size: 14px; }
input.search { font: inherit; width: 100%; padding: 10px 12px; border: 1px solid var(--line); border-radius: 8px; background: var(--panel); color: var(--ink); }
ul.results, ul.skills { list-style: none; padding: 0; margin: 8px 0 0; }
ul.results li, ul.skills li { padding: 6px 0; border-bottom: 1px solid var(--line); }
ul.results li span, ul.skills li span { color: var(--muted); font-size: 13px; margin-left: 6px; }
.grade h2 { font-size: 19px; }
.note { color: var(--muted); font-size: 14px; }