:root {
  --bg: #f4f6fa; --card: #fff; --text: #1d2433; --muted: #6b7385; --border: #dfe3ec;
  --accent: #2463eb; --ok: #15803d; --ok-bg: #e7f6ec; --bad: #b91c1c; --bad-bg: #fdecec; --learn: #d97706;
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #12151c; --card: #1b2029; --text: #e6e9ef; --muted: #98a0b3; --border: #2c3340;
    --accent: #6c9cff; --ok: #4ade80; --ok-bg: #16301f; --bad: #f87171; --bad-bg: #3a1c1c; --learn: #fbbf24;
  }
}
* { box-sizing: border-box; }
body { margin: 0; font: 16px/1.5 system-ui, -apple-system, "Segoe UI", sans-serif; background: var(--bg); color: var(--text); }
a { color: var(--accent); text-decoration: none; }
header { display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: .5rem;
  padding: .75rem 1.25rem; background: var(--card); border-bottom: 1px solid var(--border); position: sticky; top: 0; z-index: 2; }
.brand { font-weight: 700; color: var(--text); }
.brand small { color: var(--muted); font-weight: 400; }
nav a { margin-left: 1rem; color: var(--muted); }
nav a.active { color: var(--accent); font-weight: 600; }
main { max-width: 900px; margin: 1.25rem auto; padding: 0 1rem; }
footer { text-align: center; color: var(--muted); font-size: .8rem; padding: 2rem 1rem; }
.muted { color: var(--muted); }
.card { background: var(--card); border: 1px solid var(--border); border-radius: 12px; padding: 1.25rem; margin-bottom: 1rem; }
.card h3 { margin-top: 0; }
.grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(250px, 1fr)); gap: 1rem; }
.stat .big { font-size: 2.2rem; font-weight: 700; }
.stat .big.ok { color: var(--ok); }
.stat .big small { font-size: 1rem; color: var(--muted); font-weight: 400; }
.actions { display: flex; flex-direction: column; gap: .5rem; justify-content: center; }
.btn { display: inline-block; padding: .6rem 1rem; border-radius: 8px; border: 1px solid var(--border); background: var(--card);
  color: var(--text); font: inherit; cursor: pointer; text-align: center; }
.btn:hover { border-color: var(--accent); }
.btn.primary { background: var(--accent); color: #fff; border-color: var(--accent); }
button.link { background: none; border: none; color: var(--bad); cursor: pointer; font: inherit; text-decoration: underline; }
.bar { height: 10px; background: var(--border); border-radius: 5px; overflow: hidden; display: flex; margin: .4rem 0; }
.bar .m { background: var(--ok); } .bar .l { background: var(--learn); }
table.subs { width: 100%; border-collapse: collapse; }
table.subs td { padding: .35rem .5rem; border-top: 1px solid var(--border); }
table.subs td.w { width: 30%; } table.subs td.n { white-space: nowrap; color: var(--muted); font-size: .9rem; text-align: right; }
.toolbar { display: flex; flex-wrap: wrap; gap: 1rem; align-items: center; margin-bottom: 1rem; }
.toolbar select, .toolbar input { font: inherit; padding: .4rem; border-radius: 6px; border: 1px solid var(--border); background: var(--card); color: var(--text); max-width: 100%; }
.toolbar input { flex: 1; }
.qmeta { display: flex; justify-content: space-between; align-items: center; font-size: .85rem; color: var(--muted); }
.stem { font-size: 1.2rem; font-weight: 600; margin: .75rem 0 1rem; }
.tag { padding: .1rem .5rem; border-radius: 999px; background: var(--border); font-size: .8rem; }
.tag.new { background: #dbe6ff; color: #1e40af; } .tag.learning { background: #fef3c7; color: #92400e; } .tag.mastered { background: #dcfce7; color: #166534; }
ol.choices { list-style: none; padding: 0; margin: 0; display: grid; gap: .5rem; }
ol.choices button { width: 100%; text-align: left; padding: .75rem 1rem; border-radius: 8px; border: 2px solid var(--border);
  background: var(--card); color: var(--text); font: inherit; cursor: pointer; }
ol.choices button b { display: inline-block; width: 1.5rem; color: var(--muted); }
ol.choices button:hover:not(:disabled) { border-color: var(--accent); }
ol.choices button.selected { border-color: var(--accent); background: color-mix(in srgb, var(--accent) 12%, var(--card)); }
ol.choices button.right { border-color: var(--ok); background: var(--ok-bg); }
ol.choices button.wrong { border-color: var(--bad); background: var(--bad-bg); }
ol.choices button:disabled { cursor: default; color: var(--text); }
.after { margin-top: 1rem; display: flex; align-items: center; justify-content: space-between; gap: 1rem; }
.feedback.ok, p.ok, li.ok, .exams .ok { color: var(--ok); } .feedback.bad, p.bad, .exams .bad { color: var(--bad); }
.feedback { font-weight: 600; }
figure { margin: 0 0 1rem; text-align: center; }
figure img { max-width: 100%; max-height: 360px; background: #fff; border-radius: 8px; border: 1px solid var(--border); }
figcaption { font-size: .85rem; color: var(--muted); }
.dots { display: flex; flex-wrap: wrap; gap: 4px; margin-bottom: 1rem; }
.dots button { width: 2.1rem; height: 2.1rem; border-radius: 6px; border: 1px solid var(--border); background: var(--card); color: var(--muted); cursor: pointer; font-size: .8rem; }
.dots button.done { background: color-mix(in srgb, var(--accent) 20%, var(--card)); color: var(--text); }
.dots button.cur { outline: 2px solid var(--accent); }
.examnav { display: flex; gap: .5rem; justify-content: space-between; }
.result.ok { border-color: var(--ok); } .result.bad { border-color: var(--bad); }
.review .stem { font-size: 1rem; }
ol.plain { padding-left: 1.2rem; list-style: none; margin: 0; }
.ghead { margin: 1.5rem 0 .5rem; font-size: 1rem; color: var(--muted); }
.hint { font-size: .85rem; text-align: center; }
.exams { padding-left: 1.2rem; }
.tutorial { line-height: 1.65; }
.tutorial h1 { margin-top: 0; }
.tutorial h2 { margin-top: 2rem; padding-bottom: .3rem; border-bottom: 1px solid var(--border); display: flex; flex-wrap: wrap; align-items: baseline; gap: .5rem 1rem; }
.tutorial h2 .practice { font-size: .85rem; font-weight: 400; margin-left: auto; }
.tutorial hr { display: none; }
.tutorial table { border-collapse: collapse; margin: .75rem 0; width: 100%; font-size: .95rem; }
.tutorial th, .tutorial td { border: 1px solid var(--border); padding: .4rem .6rem; text-align: left; vertical-align: top; }
.tutorial th { background: color-mix(in srgb, var(--accent) 8%, var(--card)); }
.tutorial li { margin: .2rem 0; }
.tutorial code { background: var(--border); padding: 0 .3rem; border-radius: 4px; }
.steps { display: flex; flex-wrap: wrap; gap: .4rem; margin-bottom: 1rem; }
.steps a { margin: 0; padding: .3rem .75rem; border-radius: 999px; border: 1px solid var(--border); background: var(--card); color: var(--muted); font-size: .9rem; }
.steps a.done { color: var(--ok); border-color: var(--ok); }
.steps a.cur { background: var(--accent); border-color: var(--accent); color: #fff; }
.quiz-head { margin: 2rem 0 .25rem; }
.quiz-score { display: flex; justify-content: space-between; align-items: center; font-weight: 600; }