:root {
  --ink: #1b2330; --muted: #5d6675; --line: #dfe3ea; --bg: #f4f6fa; --card: #ffffff;
  --navy: #1f3a64; --navy-2: #2d5590; --gold: #f5b942; --gold-soft: #fff4d9;
  --good: #1f8a4c; --good-soft: #e5f5ec; --bad: #c2412d; --bad-soft: #fdecea;
  --radius: 16px; --shadow: 0 6px 20px rgba(23, 37, 64, .10);
}
@media (prefers-color-scheme: dark) {
  :root {
    --ink: #e8ecf3; --muted: #a3acbb; --line: #2c3442; --bg: #11151c; --card: #1a202a;
    --navy: #8fb2e8; --navy-2: #b5cdf2; --gold: #f5b942; --gold-soft: #3a2f14;
    --good: #4cc27f; --good-soft: #15301f; --bad: #f07a65; --bad-soft: #3a1c17;
    --shadow: 0 6px 20px rgba(0, 0, 0, .35);
  }
}
* { box-sizing: border-box; }
html, body { margin: 0; background: var(--bg); color: var(--ink); }
body { font: 16px/1.45 system-ui, -apple-system, "Segoe UI", Roboto, sans-serif; -webkit-tap-highlight-color: transparent; }
a { color: inherit; }
button { font: inherit; color: inherit; cursor: pointer; }

.top { position: sticky; top: 0; z-index: 5; display: flex; align-items: baseline; gap: 12px;
  padding: calc(12px + env(safe-area-inset-top)) 16px 12px; background: var(--card); border-bottom: 1px solid var(--line); }
.brand { font: 700 19px/1 Georgia, "Times New Roman", serif; text-decoration: none; color: var(--navy); }
.brand span { color: var(--gold); }
.crumb { color: var(--muted); font-size: 14px; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
main { max-width: 720px; margin: 0 auto; padding: 18px 16px calc(32px + env(safe-area-inset-bottom)); }

h1 { font: 700 26px/1.2 Georgia, "Times New Roman", serif; margin: 6px 0 4px; }
h2 { font-size: 15px; text-transform: uppercase; letter-spacing: .08em; color: var(--muted); margin: 26px 0 10px; }
.lede { color: var(--muted); margin: 0 0 18px; }

/* home */
.course { margin-top: 22px; }
.module { display: block; background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 16px; margin-bottom: 12px; box-shadow: var(--shadow); text-decoration: none; }
.module .k { font-size: 13px; font-weight: 700; color: var(--gold); text-transform: uppercase; letter-spacing: .08em; }
.module .t { font: 700 20px/1.25 Georgia, serif; margin: 2px 0 6px; color: var(--navy); }
.module .m { color: var(--muted); font-size: 14px; }
.bar { height: 6px; background: var(--line); border-radius: 3px; margin-top: 10px; overflow: hidden; }
.bar i { display: block; height: 100%; background: var(--good); border-radius: 3px; }

/* module menu */
.modes { display: grid; grid-template-columns: 1fr 1fr; gap: 12px; }
.mode { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); padding: 18px 14px;
  text-align: left; box-shadow: var(--shadow); text-decoration: none; display: block; }
.mode b { display: block; font-size: 18px; color: var(--navy); margin-bottom: 4px; }
.mode span { color: var(--muted); font-size: 14px; }
.mode .ic { font-size: 26px; display: block; margin-bottom: 6px; }
@media (max-width: 380px) { .modes { grid-template-columns: 1fr; } }

/* flashcards */
.deckbar { display: flex; justify-content: space-between; align-items: center; gap: 8px; margin-bottom: 12px; flex-wrap: wrap; }
.count { font-weight: 700; color: var(--navy); }
.chips { display: flex; gap: 6px; flex-wrap: wrap; }
.chip { border: 1px solid var(--line); background: var(--card); border-radius: 999px; padding: 6px 12px; font-size: 14px; }
.chip.on { background: var(--navy); border-color: var(--navy); color: #fff; }
@media (prefers-color-scheme: dark) { .chip.on { color: #11151c; } }

.stage { perspective: 1400px; }
.card { position: relative; min-height: 330px; transition: transform .45s cubic-bezier(.2, .7, .2, 1); transform-style: preserve-3d;
  cursor: pointer; user-select: none; -webkit-user-select: none; }
.card.flipped { transform: rotateY(180deg); }
.face { position: absolute; inset: 0; backface-visibility: hidden; -webkit-backface-visibility: hidden; border-radius: 20px;
  background: var(--card); border: 1px solid var(--line); box-shadow: var(--shadow); padding: 22px 20px;
  display: flex; flex-direction: column; justify-content: center; overflow: auto; }
.front { align-items: center; text-align: center; border-top: 6px solid var(--gold); }
.front .term { font: 700 28px/1.25 Georgia, serif; color: var(--navy); }
.hint { position: absolute; bottom: 14px; left: 0; right: 0; text-align: center; color: var(--muted); font-size: 13px; }
.back { transform: rotateY(180deg); border-top: 6px solid var(--navy); justify-content: flex-start; }
.back .lab { font-size: 12px; font-weight: 700; text-transform: uppercase; letter-spacing: .08em; color: var(--muted); margin: 0 0 4px; }
.back .def { font-size: 19px; line-height: 1.5; margin: 0 0 14px; }
.back .ex { background: var(--gold-soft); border-radius: 10px; padding: 10px 12px; font-size: 16px; margin: 0 0 10px; }
.back .src { color: var(--muted); font-size: 12px; margin-top: auto; padding-top: 6px; }
.back .bterm { font-weight: 700; color: var(--navy); margin-bottom: 10px; font-size: 15px; }
.card.known .front::after { content: "Got it"; position: absolute; top: 12px; right: 14px; font-size: 12px; font-weight: 700;
  color: var(--good); background: var(--good-soft); border-radius: 999px; padding: 3px 9px; }

.nav { display: grid; grid-template-columns: 1fr 1.3fr 1fr; gap: 10px; margin-top: 16px; }
.btn { border: 1px solid var(--line); background: var(--card); border-radius: 12px; padding: 14px 10px; font-weight: 600; min-height: 50px; }
.btn.primary { background: var(--navy); border-color: var(--navy); color: #fff; }
@media (prefers-color-scheme: dark) { .btn.primary { color: #11151c; } }
.btn.good { background: var(--good-soft); border-color: var(--good); color: var(--good); }
.btn:disabled { opacity: .4; cursor: default; }
.btn:active:not(:disabled) { transform: translateY(1px); }

/* quiz */
.qcard { background: var(--card); border: 1px solid var(--line); border-radius: 20px; box-shadow: var(--shadow); padding: 20px; }
.qkind { font-size: 12px; font-weight: 700; text-transform: uppercase; letter-spacing: .08em; color: var(--gold); margin-bottom: 8px; }
.qtext { font-size: 19px; line-height: 1.45; margin: 0 0 16px; }
.qtext.term { font: 700 24px/1.3 Georgia, serif; color: var(--navy); }
.opts { display: grid; gap: 10px; }
.opt { text-align: left; border: 1.5px solid var(--line); background: var(--card); border-radius: 12px; padding: 13px 14px;
  font-size: 16px; line-height: 1.4; display: flex; gap: 10px; align-items: flex-start; min-height: 50px; }
.opt .l { flex: 0 0 26px; height: 26px; border-radius: 50%; background: var(--bg); display: grid; place-items: center;
  font-weight: 700; font-size: 14px; color: var(--muted); }
.opt.right { border-color: var(--good); background: var(--good-soft); }
.opt.right .l { background: var(--good); color: #fff; }
.opt.wrong { border-color: var(--bad); background: var(--bad-soft); }
.opt.wrong .l { background: var(--bad); color: #fff; }
.opt:disabled { cursor: default; color: var(--ink); }
.feedback { margin-top: 14px; border-radius: 12px; padding: 12px 14px; font-size: 15px; }
.feedback.good { background: var(--good-soft); }
.feedback.bad { background: var(--bad-soft); }
.feedback b { display: block; margin-bottom: 4px; }
.progress { display: flex; gap: 4px; margin-bottom: 14px; }
.progress i { flex: 1; height: 6px; border-radius: 3px; background: var(--line); }
.progress i.r { background: var(--good); } .progress i.w { background: var(--bad); } .progress i.c { background: var(--navy); }

.score { text-align: center; padding: 26px 18px; }
.score .big { font: 700 54px/1 Georgia, serif; color: var(--navy); }
.score p { color: var(--muted); }
.missed { text-align: left; margin-top: 16px; }
.missed div { border-top: 1px solid var(--line); padding: 10px 0; font-size: 15px; }
.missed b { color: var(--navy); }
.row { display: flex; gap: 10px; margin-top: 16px; flex-wrap: wrap; }
.row .btn { flex: 1; min-width: 140px; }
.mode.wide { grid-column: 1 / -1; }
.linkbtn { background: none; border: 0; color: var(--navy-2); font-weight: 600; padding: 12px 0 0; text-decoration: underline; }
.worked { margin-top: 10px; border: 1px solid var(--line); border-radius: 12px; padding: 12px 14px; font-size: 15px; background: var(--bg); }
.worked .lab { font-size: 12px; font-weight: 700; text-transform: uppercase; letter-spacing: .06em; color: var(--muted); margin-bottom: 6px; }
.worked .wq { margin-bottom: 6px; } .worked .wa { margin-bottom: 8px; }
.worked ol { margin: 0; padding-left: 20px; } .worked li { margin-bottom: 8px; }
.worked .wm { overflow-x: auto; padding: 2px 0; } .worked .ww { color: var(--muted); font-size: 14px; }
.empty { color: var(--muted); text-align: center; padding: 40px 10px; }
.katex { font-size: 1.04em; }