:root {
  --bg: #f7f7f4; --surface: #ffffff; --ink: #1b1d22; --muted: #5f6470; --line: #e3e3dd;
  --accent: #2f5bd3; --accent-ink: #ffffff; --accent-soft: #e8eefc;
  --good: #1f7a4d; --good-soft: #e3f4ea; --bad: #b3261e; --bad-soft: #fbe6e4; --now: #c27a00; --now-soft: #fff3dc;
  --radius: 12px; color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #121317; --surface: #1b1d23; --ink: #ecedf0; --muted: #a2a6b0; --line: #2c2f37;
    --accent: #7c9cf5; --accent-ink: #0d1020; --accent-soft: #232b45;
    --good: #5fd39a; --good-soft: #17301f; --bad: #ff8a80; --bad-soft: #3a1c1a; --now: #ffbf4d; --now-soft: #3a2c10;
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #121317; --surface: #1b1d23; --ink: #ecedf0; --muted: #a2a6b0; --line: #2c2f37;
  --accent: #7c9cf5; --accent-ink: #0d1020; --accent-soft: #232b45;
  --good: #5fd39a; --good-soft: #17301f; --bad: #ff8a80; --bad-soft: #3a1c1a; --now: #ffbf4d; --now-soft: #3a2c10;
  color-scheme: dark;
}
* { box-sizing: border-box; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 16px/1.55 system-ui, -apple-system, "Segoe UI", Roboto, sans-serif; }
a { color: var(--accent); }
.top { display: flex; align-items: center; justify-content: space-between; gap: 12px; padding: 12px 16px;
  border-bottom: 1px solid var(--line); background: var(--surface); position: sticky; top: 0; z-index: 5; }
.brand { font-weight: 700; font-size: 1.15rem; color: var(--ink); text-decoration: none; }
.brand span { color: var(--accent); }
.top nav { display: flex; gap: 14px; align-items: center; font-size: .95rem; }
.who { color: var(--muted); }
.who button { margin-left: 6px; }
main { max-width: 820px; margin: 0 auto; padding: 20px 16px 64px; }
h1 { font-size: 1.6rem; margin: 4px 0 6px; }
h2 { font-size: 1.15rem; margin: 26px 0 10px; }
.sub { color: var(--muted); margin: 0 0 18px; }
.card { background: var(--surface); border: 1px solid var(--line); border-radius: var(--radius); padding: 16px; }
.grid { display: grid; gap: 12px; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); }
.course { display: block; text-decoration: none; color: inherit; }
.course:hover { border-color: var(--accent); }
.course h3 { margin: 0 0 4px; font-size: 1.05rem; }
.course .meta { color: var(--muted); font-size: .9rem; }
.pill { display: inline-block; font-size: .78rem; padding: 2px 8px; border-radius: 999px; background: var(--accent-soft); color: var(--accent); font-weight: 600; }
.pill.now { background: var(--now-soft); color: var(--now); }
.pill.soon { background: var(--line); color: var(--muted); }
.week { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
.bar { height: 8px; background: var(--line); border-radius: 99px; overflow: hidden; flex: 1; min-width: 160px; }
.bar > i { display: block; height: 100%; background: var(--accent); }
.chapters { list-style: none; padding: 0; margin: 0; display: grid; gap: 8px; }
.chapters li a { display: flex; gap: 12px; align-items: center; padding: 12px 14px; text-decoration: none; color: inherit; }
.chapters li a:hover { border-color: var(--accent); }
.chapters .n { font-weight: 700; color: var(--muted); width: 2ch; text-align: right; flex: none; }
.chapters .t { flex: 1; }
.chapters .best { font-size: .85rem; color: var(--muted); white-space: nowrap; }
.chapters li.current a { border-color: var(--now); box-shadow: 0 0 0 1px var(--now) inset; }
.q { margin: 0 0 14px; }
.q .stem { font-weight: 500; margin-bottom: 10px; }
.q .num { color: var(--muted); font-size: .85rem; margin-bottom: 4px; }
.choices { display: grid; gap: 8px; }
.choice { display: flex; gap: 10px; align-items: flex-start; text-align: left; width: 100%; padding: 10px 12px;
  border: 1px solid var(--line); border-radius: 10px; background: var(--surface); color: inherit; font: inherit; cursor: pointer; }
.choice:hover:not(:disabled) { border-color: var(--accent); }
.choice .k { font-weight: 700; color: var(--muted); flex: none; }
.choice.sel { border-color: var(--accent); background: var(--accent-soft); }
.choice.right { border-color: var(--good); background: var(--good-soft); }
.choice.wrong { border-color: var(--bad); background: var(--bad-soft); }
.choice:disabled { cursor: default; }
.explain { margin-top: 10px; padding: 10px 12px; border-left: 3px solid var(--accent); background: var(--accent-soft); border-radius: 6px; font-size: .95rem; }
.actions { display: flex; gap: 10px; flex-wrap: wrap; margin-top: 18px; }
button { font: inherit; padding: 9px 16px; border-radius: 10px; border: 1px solid var(--line); background: var(--surface); color: var(--ink); cursor: pointer; }
button.primary { background: var(--accent); color: var(--accent-ink); border-color: var(--accent); font-weight: 600; }
button:disabled { opacity: .5; cursor: default; }
.score { font-size: 2rem; font-weight: 700; }
table { width: 100%; border-collapse: collapse; font-size: .95rem; }
th, td { text-align: left; padding: 8px 6px; border-bottom: 1px solid var(--line); }
th { color: var(--muted); font-weight: 600; }
dialog { border: 1px solid var(--line); border-radius: 16px; background: var(--surface); color: var(--ink); width: min(420px, calc(100vw - 32px)); padding: 22px; }
dialog::backdrop { background: rgba(10, 12, 20, .55); }
dialog h2 { margin-top: 0; }
dialog label { display: block; font-weight: 600; margin: 12px 0 6px; }
dialog input { width: 100%; font: inherit; padding: 10px 12px; border: 1px solid var(--line); border-radius: 10px; background: var(--bg); color: var(--ink); }
dialog .row { display: flex; gap: 8px; flex-wrap: wrap; margin: 8px 0; }
.opt { font-weight: 400; color: var(--muted); }
.hint { color: var(--muted); font-size: .85rem; margin: 6px 0 0; }
.msg { min-height: 1.4em; color: var(--bad); font-size: .92rem; margin: 8px 0; }
.msg.info { color: var(--ink); }
.katex-display { overflow-x: auto; overflow-y: hidden; }
@media (max-width: 520px) { h1 { font-size: 1.35rem; } .top nav { gap: 10px; } }

/* explanations, citations, difficulty */
.refs { margin-top: 8px; font-size: .88rem; color: var(--muted); display: flex; gap: 6px; }
.refs ul { margin: 0; padding-left: 0; list-style: none; }
.lvl { float: right; letter-spacing: 2px; color: var(--accent); font-size: .8rem; }

/* sticky submit bar */
.submitbar { position: sticky; bottom: 0; z-index: 4; display: flex; align-items: center; justify-content: space-between; gap: 12px;
  margin: 18px -16px 0; padding: 12px 16px calc(12px + env(safe-area-inset-bottom)); background: var(--surface);
  border-top: 1px solid var(--line); box-shadow: 0 -6px 16px rgba(0, 0, 0, .06); }
#progress { color: var(--muted); font-size: .92rem; }

/* phone */
.q { scroll-margin-top: 64px; }
.choice { min-height: 48px; align-items: center; }
input, button { font-size: 16px; }              /* no iOS zoom on focus */
.top { padding-top: calc(10px + env(safe-area-inset-top)); }
@media (max-width: 640px) {
  main { padding: 14px 12px 40px; }
  .card { padding: 14px 12px; }
  .top { padding-left: 12px; padding-right: 12px; }
  .top nav { gap: 8px; font-size: .88rem; }
  .who { display: flex; gap: 6px; align-items: center; font-size: .9rem; }
  .who button { margin: 0; padding: 6px 10px; font-size: .85rem; }
  .grid { grid-template-columns: 1fr; }
  .chapters li a { padding: 12px; gap: 10px; flex-wrap: wrap; }
  .chapters .best { width: 100%; padding-left: calc(2ch + 10px); }
  .submitbar { margin: 18px -12px 0; padding-left: 12px; padding-right: 12px; }
  .submitbar button { flex: 1; max-width: 50%; }
  dialog { width: calc(100vw - 24px); padding: 18px; margin: auto; }
  .actions { flex-direction: column; }
  .actions button, .actions a { width: 100%; }
  .actions a button { width: 100%; }
  table { font-size: .88rem; }
  th, td { padding: 8px 4px; }
}

/* code (programming) and tables (truth tables, Karnaugh maps) inside questions */
.q pre { margin: 8px 0; padding: 10px 12px; background: var(--bg); border: 1px solid var(--line); border-radius: 8px;
  overflow-x: auto; font: 14px/1.45 ui-monospace, SFMono-Regular, Menlo, Consolas, monospace; }
.choice pre { margin: 0; padding: 6px 8px; }
.q code { font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace; font-size: .92em; }
.q .stem table, .q .explain table { width: auto; margin: 8px 0; font-size: .9rem; }
.q .stem td, .q .stem th { border: 1px solid var(--line); padding: 4px 8px; text-align: center; }
.choice > span:last-child { min-width: 0; overflow-x: auto; }