:root {
  --bg: #f6f5f0;
  --panel: #ffffff;
  --panel-2: #efeee7;
  --ink: #1c2321;
  --ink-2: #4a5350;
  --ink-3: #737b77;
  --line: #dcdad0;
  --accent: #0f6b62;
  --accent-soft: #d8ece8;
  --theory: #2f5c9c;
  --theory-soft: #e3ebf7;
  --practice: #9a5b0f;
  --practice-soft: #f6ead9;
  --good: #217a3c;
  --good-soft: #e0f1e4;
  --bad: #b3261e;
  --bad-soft: #f8e1df;
  --warn: #8a6100;
  --warn-soft: #faf0d2;
  --code-bg: #f0efe8;
  --shadow: 0 1px 2px rgba(20, 30, 25, .06), 0 2px 8px rgba(20, 30, 25, .04);
  --radius: 10px;
  --sans: "Segoe UI", system-ui, -apple-system, "Helvetica Neue", Arial, sans-serif;
  --serif: Georgia, "Times New Roman", serif;
  --mono: "Cascadia Code", Consolas, "SFMono-Regular", Menlo, monospace;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #141816;
    --panel: #1c211f;
    --panel-2: #232927;
    --ink: #e7ebe8;
    --ink-2: #b9c1bd;
    --ink-3: #8c9590;
    --line: #333b38;
    --accent: #5cc4b5;
    --accent-soft: #1d3632;
    --theory: #8fb3ec;
    --theory-soft: #1e2a3d;
    --practice: #e7b066;
    --practice-soft: #362a1a;
    --good: #72d08c;
    --good-soft: #1b3323;
    --bad: #f19a92;
    --bad-soft: #3d2120;
    --warn: #e8c766;
    --warn-soft: #362f17;
    --code-bg: #232927;
    --shadow: none;
    color-scheme: dark;
  }
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font: 16px/1.62 var(--sans);
  display: grid;
  grid-template-columns: 250px minmax(0, 1fr);
  min-height: 100vh;
}
a { color: var(--accent); }
a:hover { text-decoration-thickness: 2px; }
button, input, select { font: inherit; color: inherit; }
.skip-link { position: absolute; left: -999px; top: 0; background: var(--panel); padding: .5rem 1rem; z-index: 50; }
.skip-link:focus { left: .5rem; }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }

/* ---------- navigation ---------- */
.topbar { display: none; }
.sidebar {
  position: sticky; top: 0; height: 100vh; overflow-y: auto;
  border-right: 1px solid var(--line);
  background: var(--panel-2);
  padding: 1.25rem .9rem;
  display: flex; flex-direction: column; gap: .2rem;
}
.brand { display: flex; align-items: center; gap: .6rem; text-decoration: none; color: var(--ink); margin-bottom: 1rem; }
.brand-mark { font: 700 1.05rem var(--mono); background: var(--ink); color: var(--bg); padding: .2rem .45rem; border-radius: 6px; }
.brand-mark span { color: var(--accent); }
@media (prefers-color-scheme: dark) { .brand-mark span { color: #0f6b62; } }
.brand-name { font-weight: 650; letter-spacing: .01em; line-height: 1.15; }
.brand-sub { display: block; font-size: .64rem; font-weight: 600; letter-spacing: .08em; color: var(--ink-3); margin-top: .15rem; }
.sidebar nav { display: flex; flex-direction: column; gap: 2px; }
.sidebar nav a {
  display: flex; align-items: center; justify-content: space-between;
  padding: .42rem .7rem; border-radius: 7px; color: var(--ink-2); text-decoration: none; font-size: .95rem;
}
.sidebar nav a:hover { background: var(--panel); color: var(--ink); }
.sidebar nav a.active { background: var(--panel); color: var(--ink); font-weight: 600; box-shadow: var(--shadow); }
.nav-group { font-size: .7rem; font-weight: 700; letter-spacing: .09em; text-transform: uppercase; color: var(--ink-3); margin: 1rem .7rem .3rem; }
.nav-count { font-size: .75rem; background: var(--bad-soft); color: var(--bad); border-radius: 999px; padding: 0 .45rem; min-width: 1.4rem; text-align: center; }
.nav-count:empty { display: none; }
.side-foot { margin-top: auto; font-size: .75rem; color: var(--ink-3); padding: 1rem .7rem 0; }
.scrim { display: none; }

main { padding: 2.2rem clamp(1rem, 4vw, 3.2rem) 5rem; min-width: 0; outline: none; }
.page { max-width: 1120px; margin: 0 auto; }
.narrow { max-width: 820px; }

/* ---------- typography ---------- */
h1, h2, h3, h4 { line-height: 1.25; margin: 1.6em 0 .55em; }
h1 { font: 650 clamp(1.7rem, 3.5vw, 2.3rem)/1.2 var(--serif); margin-top: 0; letter-spacing: -.01em; }
h2 { font: 650 1.45rem/1.3 var(--serif); }
h3 { font-size: 1.12rem; }
h4 { font-size: 1rem; color: var(--ink-2); }
p, ul, ol { margin: .6em 0; }
li { margin: .2em 0; }
code { font: .9em var(--mono); background: var(--code-bg); padding: .08em .3em; border-radius: 4px; }
pre { background: var(--code-bg); padding: .9rem 1rem; border-radius: 8px; overflow-x: auto; font-size: .88rem; line-height: 1.5; }
pre code { background: none; padding: 0; }
blockquote { margin: .8em 0; padding: .1em 1em; border-left: 3px solid var(--line); color: var(--ink-2); }
hr { border: 0; border-top: 1px solid var(--line); margin: 2rem 0; }
.lede { font-size: 1.08rem; color: var(--ink-2); max-width: 70ch; }
.muted { color: var(--ink-3); }
.small { font-size: .86rem; }
.eyebrow { font-size: .74rem; font-weight: 700; letter-spacing: .09em; text-transform: uppercase; color: var(--ink-3); margin: 0 0 .4rem; }

table { border-collapse: collapse; margin: .9em 0; font-size: .93rem; display: block; overflow-x: auto; max-width: 100%; }
th, td { border: 1px solid var(--line); padding: .35rem .65rem; text-align: left; vertical-align: top; }
th { background: var(--panel-2); font-weight: 600; }

.katex { font-size: 1.07em; }
.katex-display { overflow-x: auto; overflow-y: hidden; padding: .35rem 0; margin: .6em 0; }

/* ---------- home ---------- */
.hero { display: grid; gap: .4rem; margin-bottom: 2rem; }
.split { display: grid; grid-template-columns: repeat(auto-fit, minmax(290px, 1fr)); gap: 1.1rem; }
.big-card {
  display: block; text-decoration: none; color: var(--ink);
  background: var(--panel); border: 1px solid var(--line); border-radius: 14px; padding: 1.4rem 1.5rem; box-shadow: var(--shadow);
  border-top: 5px solid var(--theory);
}
.big-card.practice { border-top-color: var(--practice); }
.big-card h2 { margin: .2rem 0 .5rem; }
.big-card ul { padding-left: 1.1rem; color: var(--ink-2); font-size: .95rem; }
.big-card:hover { border-color: var(--ink-3); }
.big-card .go { font-weight: 600; color: var(--accent); }
.stats { display: flex; flex-wrap: wrap; gap: .6rem 1.6rem; margin: 1.4rem 0 0; }
.stat b { display: block; font: 650 1.5rem var(--serif); }
.stat span { font-size: .8rem; color: var(--ink-3); }

.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(250px, 1fr)); gap: .9rem; }
.card {
  background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 1rem 1.1rem; text-decoration: none; color: var(--ink); display: flex; flex-direction: column; gap: .3rem;
  box-shadow: var(--shadow);
}
a.card:hover { border-color: var(--ink-3); }
.card h3 { margin: 0; font-size: 1.02rem; }
.card p { margin: 0; font-size: .9rem; color: var(--ink-2); }
.card .num { font: 700 .74rem var(--mono); color: var(--ink-3); letter-spacing: .05em; }
.bar { height: 6px; background: var(--panel-2); border-radius: 99px; overflow: hidden; margin-top: .4rem; }
.bar i { display: block; height: 100%; background: var(--good); }

/* ---------- theory ---------- */
.theory-layout { display: grid; grid-template-columns: minmax(0, 1fr) 230px; gap: 2.5rem; align-items: start; }
.toc { position: sticky; top: 1.5rem; font-size: .86rem; border-left: 2px solid var(--line); padding-left: .9rem; max-height: calc(100vh - 3rem); overflow-y: auto; }
.toc b { display: block; font-size: .7rem; letter-spacing: .09em; text-transform: uppercase; color: var(--ink-3); margin-bottom: .4rem; }
.toc a { display: block; color: var(--ink-2); text-decoration: none; padding: .18rem 0; }
.toc a:hover, .toc a.on { color: var(--accent); }
.theory-section { scroll-margin-top: 1rem; }
.theory-section > h2 { border-bottom: 1px solid var(--line); padding-bottom: .35rem; }
.theory-body { max-width: 78ch; }
.pager { display: flex; justify-content: space-between; gap: 1rem; margin-top: 3rem; border-top: 1px solid var(--line); padding-top: 1rem; }
.pager a { text-decoration: none; font-weight: 600; }
.pager small { display: block; color: var(--ink-3); font-weight: 400; }
.practice-link { display: inline-flex; gap: .4rem; align-items: center; background: var(--practice-soft); color: var(--practice); border-radius: 999px; padding: .25rem .8rem; font-size: .88rem; font-weight: 600; text-decoration: none; }

.box { border: 1px solid var(--line); border-left-width: 4px; border-radius: 8px; background: var(--panel); margin: 1rem 0; box-shadow: var(--shadow); }
.box-head, .box > summary { display: flex; flex-wrap: wrap; gap: .25rem .6rem; align-items: baseline; padding: .55rem .9rem .1rem; }
.box > summary { cursor: pointer; list-style: none; padding-bottom: .55rem; }
.box[open] > summary { padding-bottom: .1rem; }
.box > summary::-webkit-details-marker { display: none; }
.box > summary::before { content: "▸"; color: var(--ink-3); font-size: .8rem; }
.box[open] > summary::before { content: "▾"; }
.box-kind { font-size: .7rem; font-weight: 700; letter-spacing: .08em; text-transform: uppercase; }
.box-title { font-weight: 600; }
.box-body { padding: .1rem .95rem .6rem; }
.box-body > :first-child { margin-top: .3rem; }
.box-formula { border-left-color: var(--theory); background: var(--theory-soft); }
.box-formula .box-kind { color: var(--theory); }
.box-formula .katex-display { font-size: 1.08em; }
.box-read { border-left-color: var(--accent); }
.box-read .box-kind { color: var(--accent); }
.box-steps, .box-example { border-left-color: var(--practice); }
.box-steps .box-kind, .box-example .box-kind { color: var(--practice); }
.box-tip, .box-intuition, .box-note { border-left-color: var(--good); }
.box-tip .box-kind, .box-intuition .box-kind, .box-note .box-kind { color: var(--good); }
.box-pitfall { border-left-color: var(--bad); }
.box-pitfall .box-kind { color: var(--bad); }
.box-exam { border-left-color: var(--warn); background: var(--warn-soft); }
.box-exam .box-kind { color: var(--warn); }
.box .box { box-shadow: none; }

/* ---------- practice ---------- */
.toolbar { display: flex; flex-wrap: wrap; gap: .5rem; align-items: center; margin: 1rem 0 1.4rem; }
.chip-btn {
  border: 1px solid var(--line); background: var(--panel); border-radius: 999px; padding: .28rem .8rem; cursor: pointer; font-size: .86rem; color: var(--ink-2);
}
.chip-btn[aria-pressed="true"] { background: var(--ink); color: var(--bg); border-color: var(--ink); }
.toolbar .spacer { flex: 1; }
.q {
  background: var(--panel); border: 1px solid var(--line); border-radius: 12px; padding: 1.1rem 1.25rem 1rem; margin-bottom: 1.1rem; box-shadow: var(--shadow);
  scroll-margin-top: 1rem;
}
.q.correct { border-left: 5px solid var(--good); }
.q.wrong { border-left: 5px solid var(--bad); }
.q-meta { display: flex; flex-wrap: wrap; gap: .4rem; align-items: center; margin-bottom: .5rem; font-size: .78rem; }
.tag { border-radius: 999px; padding: .08rem .6rem; font-weight: 600; background: var(--panel-2); color: var(--ink-2); }
.tag.hw { background: var(--practice-soft); color: var(--practice); }
.tag.variant { background: var(--warn-soft); color: var(--warn); }
.tag.slide { background: var(--theory-soft); color: var(--theory); }
.tag.type { background: var(--accent-soft); color: var(--accent); }
.q-num { font: 700 .8rem var(--mono); color: var(--ink-3); }
.bookmark { margin-left: auto; border: 0; background: none; cursor: pointer; font-size: 1.1rem; color: var(--ink-3); padding: 0 .2rem; }
.bookmark[aria-pressed="true"] { color: var(--warn); }
.q-prompt > :first-child { margin-top: 0; }
.options { display: grid; gap: .45rem; margin: .8rem 0; }
.opt {
  display: flex; gap: .65rem; align-items: flex-start; border: 1px solid var(--line); border-radius: 8px; padding: .5rem .75rem; cursor: pointer; background: var(--bg);
}
.opt:hover { border-color: var(--ink-3); }
.opt input { margin-top: .35rem; accent-color: var(--accent); }
.opt .opt-letter { font: 700 .85rem var(--mono); color: var(--ink-3); margin-top: .12rem; }
.opt.is-right { border-color: var(--good); background: var(--good-soft); }
.opt.is-wrong { border-color: var(--bad); background: var(--bad-soft); }
.opt p { margin: 0; }
.fields { display: flex; flex-wrap: wrap; gap: .7rem 1.2rem; margin: .9rem 0; }
.field { display: flex; flex-direction: column; gap: .2rem; font-size: .9rem; }
.field input {
  width: 9.5rem; padding: .4rem .55rem; border: 1px solid var(--line); border-radius: 7px; background: var(--bg); font-family: var(--mono);
}
.field input.ok { border-color: var(--good); background: var(--good-soft); }
.field input.no { border-color: var(--bad); background: var(--bad-soft); }
.field .expect { font-size: .8rem; color: var(--ink-3); }
.actions { display: flex; flex-wrap: wrap; gap: .5rem; align-items: center; margin-top: .6rem; }
.btn {
  border: 1px solid var(--ink); background: var(--ink); color: var(--bg); border-radius: 8px; padding: .42rem .95rem; cursor: pointer; font-weight: 600; font-size: .92rem;
}
.btn.ghost { background: transparent; color: var(--ink); border-color: var(--line); }
.btn.ghost:hover { border-color: var(--ink-3); }
.btn:disabled { opacity: .5; cursor: default; }
.verdict { font-weight: 600; font-size: .92rem; }
.verdict.ok { color: var(--good); }
.verdict.no { color: var(--bad); }
.solution { margin-top: .9rem; border-top: 1px dashed var(--line); padding-top: .7rem; }
.solution > h4:first-child { margin-top: 0; }
.solution-label { font-size: .72rem; font-weight: 700; letter-spacing: .08em; text-transform: uppercase; color: var(--accent); }
.selfmark { display: flex; flex-wrap: wrap; gap: .5rem; align-items: center; margin-top: .8rem; font-size: .9rem; }

/* ---------- drills ---------- */
.drill-head { display: flex; flex-wrap: wrap; gap: .6rem; align-items: center; justify-content: space-between; }
.drill-steps ol { padding-left: 1.3rem; }
.drill-steps li { margin: .5rem 0; }

/* ---------- exam ---------- */
.exam-setup { background: var(--panel); border: 1px solid var(--line); border-radius: 12px; padding: 1.2rem 1.4rem; max-width: 640px; }
.exam-setup label { display: flex; gap: .6rem; align-items: center; margin: .5rem 0; }
.exam-setup select { padding: .3rem .5rem; border-radius: 7px; border: 1px solid var(--line); background: var(--bg); }
.timer { position: sticky; top: .6rem; z-index: 5; display: flex; justify-content: space-between; align-items: center; gap: 1rem; background: var(--ink); color: var(--bg); border-radius: 10px; padding: .55rem 1rem; margin-bottom: 1rem; font-weight: 600; }
.timer .t { font: 700 1.1rem var(--mono); }
.timer.low { background: var(--bad); }
.score-card { background: var(--panel); border: 1px solid var(--line); border-radius: 12px; padding: 1.2rem 1.4rem; margin-bottom: 1.5rem; }
.score-card .big { font: 650 2.4rem var(--serif); }
.history { font-size: .88rem; }

.empty { padding: 2rem; text-align: center; color: var(--ink-3); border: 1px dashed var(--line); border-radius: 12px; }

/* ---------- responsive ---------- */
@media (max-width: 1100px) {
  .theory-layout { grid-template-columns: minmax(0, 1fr); }
  .toc { position: static; border-left: 0; padding: .8rem 1rem; background: var(--panel); border: 1px solid var(--line); border-radius: 10px; max-height: none; }
  .toc a { display: inline-block; margin-right: .9rem; }
}
@media (max-width: 860px) {
  body { grid-template-columns: minmax(0, 1fr); }
  .topbar {
    display: flex; align-items: center; gap: .8rem; position: sticky; top: 0; z-index: 30;
    background: var(--panel-2); border-bottom: 1px solid var(--line); padding: .55rem 16px;
  }
  .topbar .brand { margin: 0; }
  .menu-btn { border: 1px solid var(--line); background: var(--panel); border-radius: 8px; width: 2.4rem; height: 2.2rem; cursor: pointer; font-size: 1.1rem; }
  .sidebar { position: fixed; z-index: 40; left: 0; top: 0; width: 270px; transform: translateX(-100%); transition: transform .2s ease; }
  .sidebar.open { transform: none; }
  .brand-side { display: none; }
  .scrim { display: block; position: fixed; inset: 0; background: rgba(0,0,0,.35); z-index: 35; }
  .scrim[hidden] { display: none; }
  main { padding: 1.2rem 16px 4rem; }
  .q { padding: .95rem 1rem; }
  .field input { width: 8rem; }
}
@media print {
  .sidebar, .topbar, .toc, .pager, .actions, .toolbar { display: none !important; }
  body { display: block; background: #fff; color: #000; }
  details.box > .box-body { display: block; }
}