:root {
  --ink: #1b1b1f;
  --muted: #5b5d66;
  --line: #d9dae0;
  --paper: #ffffff;
  --bg: #f3f4f7;
  --accent: #1f5f8b;
  --accent-ink: #ffffff;
  --soft: #e8f0f6;
  --c: #2e7d4f;
  --a: #a3480f;
  color-scheme: light;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font: 16px/1.5 system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
}

button, input, select { font: inherit; }

button {
  border: 1px solid var(--line);
  background: var(--paper);
  color: var(--ink);
  border-radius: 6px;
  padding: 0.4rem 0.8rem;
  cursor: pointer;
}
button:hover { border-color: var(--accent); }
button:focus-visible, input:focus-visible, select:focus-visible, summary:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}
button.primary {
  background: var(--accent);
  color: var(--accent-ink);
  border-color: var(--accent);
  width: 100%;
  padding: 0.6rem;
  font-weight: 600;
}

.site-header {
  padding: 1rem 1.5rem 0.5rem;
}
.site-header h1 { margin: 0; font-size: 1.4rem; }
.site-header p { margin: 0.2rem 0 0; color: var(--muted); }

.layout {
  display: grid;
  grid-template-columns: minmax(280px, 340px) 1fr;
  gap: 1.25rem;
  padding: 1rem 1.5rem 3rem;
  align-items: start;
}

/* ---------- builder ---------- */
.builder {
  background: var(--paper);
  border: 1px solid var(--line);
  border-radius: 10px;
  padding: 1rem;
  position: sticky;
  top: 1rem;
  max-height: calc(100vh - 2rem);
  overflow: auto;
}
.builder h2 { margin: 0 0 0.75rem; font-size: 1.1rem; }
.field { display: block; margin-bottom: 0.9rem; font-weight: 600; font-size: 0.9rem; }
.field input[type="text"], .field select { display: block; width: 100%; margin-top: 0.25rem; padding: 0.4rem; border: 1px solid var(--line); border-radius: 6px; font-weight: 400; }
.field small { display: block; font-weight: 400; color: var(--muted); margin-top: 0.25rem; }
.seed-row { display: flex; gap: 0.4rem; margin-top: 0.25rem; }
.seed-row input { margin-top: 0 !important; font-family: ui-monospace, Consolas, monospace; }
.check { display: block; margin-bottom: 0.9rem; font-size: 0.9rem; }

.topics { border: 1px solid var(--line); border-radius: 8px; padding: 0.5rem 0.75rem 0.25rem; margin: 0 0 0.9rem; }
.topics legend { font-weight: 600; font-size: 0.9rem; padding: 0 0.25rem; }
.topic { border-bottom: 1px solid var(--line); padding: 0.5rem 0; }
.topic:last-child { border-bottom: 0; }
.topic-row { display: flex; align-items: center; justify-content: space-between; gap: 0.5rem; }
.topic-row label { font-size: 0.95rem; }
.topic-row input { width: 4rem; padding: 0.3rem; border: 1px solid var(--line); border-radius: 6px; text-align: center; }
.topic details { margin-top: 0.25rem; font-size: 0.85rem; }
.topic summary { cursor: pointer; color: var(--accent); }
.topic ul { list-style: none; margin: 0.4rem 0 0; padding: 0; }
.topic li { margin: 0.2rem 0; }
.topic li.filtered { opacity: 0.45; }
.lvl { display: inline-block; font-size: 0.7rem; font-weight: 700; padding: 0 0.3rem; border-radius: 4px; border: 1px solid currentColor; margin-left: 0.2rem; }
.lvl-C { color: var(--c); }
.lvl-A { color: var(--a); }
.lvl-CA { color: var(--muted); }

/* ---------- toolbar ---------- */
.toolbar { display: flex; flex-wrap: wrap; gap: 0.5rem; align-items: center; margin-bottom: 0.4rem; }
.toolbar .spacer { flex: 1; }
.print-tip { margin: 0 0 0.75rem; font-size: 0.85rem; color: var(--muted); }

/* ---------- worksheet (looks like a paper on screen) ---------- */
.sheet {
  background: var(--paper);
  border: 1px solid var(--line);
  border-radius: 4px;
  padding: 2rem 2.5rem;
  max-width: 820px;
  font-family: "Times New Roman", Times, serif;
  font-size: 17px;
}
#answers { display: none; }
.sheet-head { border-bottom: 2px solid var(--ink); margin-bottom: 1.25rem; padding-bottom: 0.6rem; }
.sheet-title { margin: 0 0 0.4rem; font-size: 1.4rem; }
.sheet-meta { display: flex; flex-wrap: wrap; gap: 1rem 2rem; font-size: 0.95rem; }
.sheet-meta .total { margin-left: auto; font-weight: 700; }
.g-note { font-size: 0.9rem; margin: 0.6rem 0 0; }

.q { display: grid; grid-template-columns: 2.2rem 1fr; margin-bottom: 1.6rem; }
.qnum { font-weight: 700; }
.intro p { margin: 0 0 0.5rem; }
.intro { margin-bottom: 0.5rem; }
.fig { margin: 0.5rem 0 0.75rem; text-align: center; }
.diagram { max-width: 100%; height: auto; color: var(--ink); }

.part, .apart { display: grid; grid-template-columns: 2.2rem 1fr 2.5rem; align-items: start; margin: 0.35rem 0; }
.part.single { grid-template-columns: 1fr 2.5rem; }
.plabel { font-weight: 400; }
.marks { text-align: right; font-weight: 700; }
.hide-marks .marks { display: none; }

.toggle { margin-top: 0.5rem; font-family: system-ui, sans-serif; font-size: 0.85rem; padding: 0.2rem 0.6rem; }
.inline-answer {
  margin-top: 0.6rem;
  background: var(--soft);
  border-left: 3px solid var(--accent);
  padding: 0.6rem 0.8rem;
  border-radius: 0 6px 6px 0;
}
.apart .abody { min-width: 0; }
.apart:only-child { grid-template-columns: 1fr 2.5rem; }
.apart:only-child .plabel { display: none; }
.wline { margin: 0.15rem 0; max-width: 100%; }
.final { margin-top: 0.3rem; font-weight: 700; }
.final-label { font-weight: 400; font-style: italic; }
.empty, .error { font-family: system-ui, sans-serif; color: var(--muted); }
.error { color: #b00020; }

.katex { font-size: 1.05em; }
.katex-display { margin: 0.4rem 0; overflow-x: auto; overflow-y: hidden; }

/* ---------- narrow screens ---------- */
@media (max-width: 860px) {
  .layout { grid-template-columns: 1fr; padding: 0.75rem 16px 2rem; }
  .builder { position: static; max-height: none; }
  .sheet { padding: 1.25rem 1rem; }
  .site-header { padding: 1rem 16px 0; }
}

/* ---------- printing ---------- */
@page { size: A4; margin: 16mm 15mm; }

@media print {
  body { background: #fff; font-size: 12pt; }
  .no-print, .builder, .site-header, .toolbar, .print-tip, .toggle, .inline-answer { display: none !important; }
  .layout { display: block; padding: 0; }
  .sheet { border: 0; padding: 0; max-width: none; font-size: 12pt; }
  .q { break-inside: avoid; page-break-inside: avoid; }
  .fig { break-inside: avoid; }

  /* default print (Ctrl+P) = questions only */
  #answers { display: none; }
  body.print-answers #questions { display: none; }
  body.print-answers #answers { display: block; }
  body.print-both #answers { display: block; break-before: page; page-break-before: always; }
  .answer-q { margin-bottom: 1.1rem; }
  .katex-display, .wline { overflow: visible !important; }
}