:root {
  --bg: #0f1220;
  --bg-soft: #171b2e;
  --card: #1c2138;
  --card-2: #232945;
  --line: #2f3760;
  --txt: #e8ebf7;
  --txt-dim: #a6aecb;
  --accent: #6ea8ff;
  --accent-2: #7ef0c0;
  --warn: #ffc46b;
  --danger: #ff8080;
  --pink: #ff9ed4;
  --violet: #c4a6ff;
  --radius: 14px;
  --mono: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--txt);
  font-family: "Segoe UI", system-ui, -apple-system, Roboto, Helvetica, Arial, sans-serif;
  font-size: 16.5px;
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
}

/* ---------- Layout ---------- */
.wrap { display: flex; min-height: 100vh; }

.sidebar {
  width: 268px;
  flex: 0 0 268px;
  background: var(--bg-soft);
  border-right: 1px solid var(--line);
  padding: 22px 16px 40px;
  position: sticky;
  top: 0;
  height: 100vh;
  overflow-y: auto;
}

.brand { display: flex; align-items: center; gap: 10px; margin-bottom: 6px; }
.brand .logo {
  width: 36px; height: 36px; border-radius: 10px;
  background: linear-gradient(135deg, var(--accent), var(--violet));
  display: grid; place-items: center; font-weight: 800; color: #0f1220; font-size: 18px;
}
.brand b { font-size: 16px; letter-spacing: .2px; }
.brand small { display: block; color: var(--txt-dim); font-size: 11.5px; letter-spacing: .4px; text-transform: uppercase; }

.nav-group-title {
  margin: 20px 0 8px; font-size: 11px; letter-spacing: 1.2px;
  text-transform: uppercase; color: var(--txt-dim);
}
.sidebar a {
  display: block; padding: 8px 11px; border-radius: 9px;
  color: var(--txt-dim); text-decoration: none; font-size: 14.5px;
  border-left: 3px solid transparent;
}
.sidebar a:hover { background: var(--card); color: var(--txt); }
.sidebar a.active {
  background: var(--card-2); color: var(--txt);
  border-left-color: var(--accent); font-weight: 600;
}
.sidebar a .n { color: var(--accent); font-family: var(--mono); margin-right: 6px; }

.main { flex: 1; min-width: 0; padding: 34px 40px 100px; max-width: 1000px; }

/* ---------- Typography ---------- */
h1 { font-size: 2.05rem; line-height: 1.2; margin: 0 0 6px; }
h2 {
  font-size: 1.5rem; margin: 46px 0 12px; padding-bottom: 8px;
  border-bottom: 1px solid var(--line);
  scroll-margin-top: 20px;
}
h3 { font-size: 1.16rem; margin: 28px 0 8px; color: var(--accent-2); }
h4 { font-size: 1rem; margin: 18px 0 6px; color: var(--violet); }
p { margin: 10px 0; }
a { color: var(--accent); }
code {
  font-family: var(--mono); font-size: .88em;
  background: var(--card-2); padding: 2px 6px; border-radius: 6px;
  border: 1px solid var(--line); color: var(--accent-2);
}
ul, ol { padding-left: 24px; }
li { margin: 5px 0; }
strong { color: #fff; }
hr { border: 0; border-top: 1px solid var(--line); margin: 34px 0; }

.lede { color: var(--txt-dim); font-size: 1.05rem; margin: 4px 0 18px; }

/* ---------- Header bar ---------- */
.topbar {
  display: flex; align-items: center; gap: 10px; flex-wrap: wrap;
  margin-bottom: 20px;
}
.pill {
  font-size: 12px; padding: 3px 11px; border-radius: 999px;
  background: var(--card-2); border: 1px solid var(--line); color: var(--txt-dim);
}

/* ---------- Cards ---------- */
.card {
  background: var(--card); border: 1px solid var(--line);
  border-radius: var(--radius); padding: 18px 20px; margin: 18px 0;
}
.card.tight { padding: 12px 16px; }
.grid2 { display: grid; grid-template-columns: 1fr 1fr; gap: 16px; }
.grid3 { display: grid; grid-template-columns: repeat(3, 1fr); gap: 14px; }
@media (max-width: 900px) { .grid2, .grid3 { grid-template-columns: 1fr; } }

.card h3:first-child, .card h4:first-child { margin-top: 0; }

/* ---------- Callouts ---------- */
.box { border-radius: 12px; padding: 14px 18px; margin: 18px 0; border: 1px solid; }
.box .tag {
  display: inline-block; font-size: 11px; font-weight: 700; letter-spacing: 1px;
  text-transform: uppercase; margin-bottom: 6px;
}
.box p:first-of-type { margin-top: 0; }
.box p:last-child { margin-bottom: 0; }

.tip    { background: rgba(126, 240, 192, .07); border-color: rgba(126, 240, 192, .35); }
.tip .tag { color: var(--accent-2); }

.trick  { background: rgba(196, 166, 255, .08); border-color: rgba(196, 166, 255, .4); }
.trick .tag { color: var(--violet); }

.warn   { background: rgba(255, 196, 107, .07); border-color: rgba(255, 196, 107, .35); }
.warn .tag { color: var(--warn); }

.care   { background: rgba(255, 128, 128, .07); border-color: rgba(255, 128, 128, .35); }
.care .tag { color: var(--danger); }

.plain  { background: var(--card-2); border-color: var(--line); }
.plain .tag { color: var(--accent); }

/* ---------- Formula blocks ---------- */
.formula {
  background: #12162a; border: 1px solid var(--line); border-left: 4px solid var(--accent);
  border-radius: 10px; padding: 12px 16px; margin: 14px 0;
  font-family: var(--mono); font-size: .95rem; overflow-x: auto; color: #dfe6ff;
}
.formula .lbl { color: var(--txt-dim); font-size: 12px; display: block; margin-bottom: 4px; font-family: inherit; }
.formula.pink { border-left-color: var(--pink); }
.formula.green { border-left-color: var(--accent-2); }
.formula.violet { border-left-color: var(--violet); }

.math { font-family: var(--mono); color: var(--accent-2); }

/* ---------- Tables ---------- */
table { border-collapse: collapse; width: 100%; margin: 16px 0; font-size: .94rem; }
th, td { border: 1px solid var(--line); padding: 9px 12px; text-align: left; vertical-align: top; }
th { background: var(--card-2); font-weight: 600; color: #fff; }
tbody tr:nth-child(even) { background: rgba(255,255,255,.02); }
td.c, th.c { text-align: center; }
.scroll-x { overflow-x: auto; }

/* ---------- Figures ---------- */
figure {
  margin: 22px 0; padding: 16px; background: var(--card);
  border: 1px solid var(--line); border-radius: var(--radius);
  text-align: center;
}
figure svg { max-width: 100%; height: auto; display: block; margin: 0 auto; }
figcaption { color: var(--txt-dim); font-size: 13.5px; margin-top: 12px; }
.fig-row { display: flex; gap: 14px; flex-wrap: wrap; justify-content: center; }
.fig-row figure { flex: 1 1 260px; margin: 14px 0; }

/* SVG helper classes */
.node { fill: var(--card-2); stroke: var(--accent); stroke-width: 2; }
.node-hot { fill: var(--danger); stroke: #fff; stroke-width: 2; }
.node-ok  { fill: var(--accent-2); stroke: #0f1220; }
.node-mut { fill: #4a5280; stroke: #8f98c4; }
.lbl { fill: var(--txt); font-size: 14px; font-family: inherit; text-anchor: middle; dominant-baseline: middle; }
.lbl-s { fill: var(--txt-dim); font-size: 12.5px; text-anchor: middle; font-family: inherit; }
.edge { stroke: var(--txt-dim); stroke-width: 2.2; fill: none; }
.edge-hi { stroke: var(--accent-2); stroke-width: 3.2; fill: none; }
.edge-off { stroke: #454c72; stroke-width: 1.6; fill: none; stroke-dasharray: 5 5; }
.axis { stroke: var(--txt-dim); stroke-width: 1.6; }
.tick { stroke: var(--txt-dim); stroke-width: 1.2; }
.curve { fill: none; stroke: var(--accent); stroke-width: 2.6; }
.curve2 { fill: none; stroke: var(--pink); stroke-width: 2.6; }
.area2 { fill: rgba(255,158,212,.18); stroke: none; }
.dash { stroke: var(--warn); stroke-width: 1.8; stroke-dasharray: 6 5; fill: none; }
.anno { fill: var(--warn); font-size: 12.5px; font-family: inherit; }
.anno-g { fill: var(--accent-2); font-size: 12.5px; font-family: inherit; }

/* ---------- Steps ---------- */
.steps { counter-reset: s; list-style: none; padding: 0; }
.steps > li {
  counter-increment: s; position: relative; padding-left: 42px; margin: 14px 0;
}
.steps > li::before {
  content: counter(s); position: absolute; left: 0; top: 0;
  width: 28px; height: 28px; border-radius: 8px;
  background: var(--card-2); border: 1px solid var(--accent);
  color: var(--accent); display: grid; place-items: center;
  font-weight: 700; font-size: 14px; font-family: var(--mono);
}

/* ---------- Worked example ---------- */
.work { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); overflow: hidden; margin: 18px 0; }
.work .wh {
  background: var(--card-2); padding: 10px 16px; font-weight: 600; font-size: .96rem;
  border-bottom: 1px solid var(--line);
}
.work .wb { padding: 14px 18px; }
.work .wb p:first-child { margin-top: 0; }
.work .ans {
  background: rgba(126,240,192,.08); border: 1px dashed rgba(126,240,192,.4);
  border-radius: 10px; padding: 10px 14px; margin-top: 12px; font-weight: 600;
}

/* ---------- Cheat / mnem ---------- */
.mnem { display: flex; flex-wrap: wrap; gap: 8px; margin: 10px 0; }
.mnem span {
  background: var(--card-2); border: 1px solid var(--violet); color: var(--violet);
  border-radius: 8px; padding: 5px 11px; font-weight: 700; letter-spacing: .5px;
}

/* ---------- Prev/Next ---------- */
.pager { display: flex; gap: 12px; margin-top: 54px; flex-wrap: wrap; }
.pager a {
  flex: 1 1 220px; text-decoration: none; color: var(--txt);
  background: var(--card); border: 1px solid var(--line);
  border-radius: 12px; padding: 13px 16px;
}
.pager a:hover { border-color: var(--accent); }
.pager a small { display: block; color: var(--txt-dim); font-size: 12px; }
.pager a.next { text-align: right; }

/* ---------- TOC in-page ---------- */
.toc {
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 14px 20px; margin: 22px 0;
}
.toc b { display: block; margin-bottom: 8px; color: var(--accent); font-size: 13px; letter-spacing: 1px; text-transform: uppercase; }
.toc ol { margin: 0; padding-left: 20px; columns: 2; font-size: .93rem; }
.toc a { text-decoration: none; }
.toc a:hover { text-decoration: underline; }
@media (max-width: 700px) { .toc ol { columns: 1; } }

/* ---------- Checkbox quiz ---------- */
.quiz { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); padding: 16px 20px; margin: 20px 0; }
.quiz summary { cursor: pointer; font-weight: 600; color: var(--warn); }
.quiz .a { margin-top: 12px; padding: 10px 14px; background: rgba(255,196,107,.07); border-radius: 9px; border: 1px solid rgba(255,196,107,.3); }
.quiz .q { margin: 12px 0 4px; font-weight: 600; }


@media (max-width: 860px) {
  .wrap { flex-direction: column; }
  .sidebar { position: static; width: 100%; flex: none; height: auto; border-right: 0; border-bottom: 1px solid var(--line); }
  .main { padding: 24px 18px 70px; }
}

/* ---------- Print ---------- */
@media print {
  .sidebar, .pager, #topBtn { display: none !important; }
  .wrap { display: block; }
  .main { max-width: none; padding: 0; }
  body { background: #fff; color: #111; font-size: 11pt; }
  h1, h2, h3, h4, strong, code, .formula { color: #000 !important; }
  .card, .box, .formula, table, figure, .work, .toc { border-color: #bbb !important; background: #fff !important; }
  figure svg { max-width: 100%; }
  a { color: #000; text-decoration: none; }
}

figure.mb { margin-bottom: 10px; }