:root {
  --ink: #1a2744;
  --soft: #5b6780;
  --paper: #f4f1ea;
  --card: #fffdf8;
  --navy: #1a365d;
  --teal: #2c7a7b;
  --good: #2f7a4f;
  --good-bg: #e7f4ec;
  --bad: #b3372c;
  --bad-bg: #fbecea;
  --line: #e4ddd2;
  --radius: 14px;
}

* { box-sizing: border-box; }
html, body { margin: 0; }
body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: var(--paper);
  color: var(--ink);
  line-height: 1.5;
  min-height: 100vh;
}

.top {
  background: var(--navy);
  color: #f7f3ea;
  padding: 1.4rem 1.2rem 1.2rem;
}
.kicker {
  margin: 0 0 .2rem;
  letter-spacing: .08em;
  text-transform: uppercase;
  font-size: .72rem;
  opacity: .8;
}
.top h1 { margin: 0 0 .35rem; font-size: 1.45rem; }
.sub { margin: 0; color: #d7deea; font-size: .95rem; }

.tabs {
  display: flex;
  gap: .4rem;
  padding: .7rem 1rem;
  background: #fff;
  border-bottom: 1px solid var(--line);
  position: sticky;
  top: 0;
  z-index: 2;
}
.tab {
  flex: 1;
  border: 0;
  background: transparent;
  padding: .55rem .4rem;
  border-radius: 999px;
  font-weight: 700;
  color: var(--soft);
  cursor: pointer;
}
.tab.on { background: var(--navy); color: #fff; }

main { max-width: 720px; margin: 0 auto; padding: 1.1rem; }
.view { display: none; }
.view.on { display: block; }

.card, .qcard, .flash {
  background: var(--card);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 1.1rem 1.15rem;
  margin-bottom: 1rem;
}
h2 { margin: 0 0 .6rem; font-size: 1.15rem; }
h3 { margin: 1.1rem 0 .4rem; font-size: 1rem; color: var(--navy); }
ol, ul { padding-left: 1.2rem; }
.note, .hint, .src {
  color: var(--soft);
  font-size: .88rem;
}
.src { margin: .4rem 0 0; }
.row { display: flex; flex-wrap: wrap; gap: .6rem; margin-top: 1rem; }

.btn {
  border: 1px solid var(--navy);
  background: #fff;
  color: var(--navy);
  font-weight: 700;
  padding: .65rem 1rem;
  border-radius: 999px;
  cursor: pointer;
  text-decoration: none;
  display: inline-block;
}
.btn.primary { background: var(--teal); border-color: var(--teal); color: #fff; }
.btn:disabled { opacity: .45; cursor: default; }

.fact {
  background: #eef6f6;
  border-radius: 10px;
  padding: .7rem .85rem;
  margin: .55rem 0;
}
.fact strong { display: block; color: var(--navy); }
.pair {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: .5rem;
  margin: .5rem 0;
}
@media (max-width: 520px) {
  .pair { grid-template-columns: 1fr; }
}
.pair div {
  background: #f7f4ee;
  border-radius: 10px;
  padding: .65rem .75rem;
  font-size: .95rem;
}
.pair .label {
  font-weight: 800;
  color: var(--teal);
  font-size: .78rem;
  letter-spacing: .04em;
  text-transform: uppercase;
}

.video {
  position: relative;
  padding-bottom: 56.25%;
  height: 0;
  overflow: hidden;
  border-radius: 10px;
  background: #000;
  margin: .6rem 0 .2rem;
}
.video iframe {
  position: absolute;
  inset: 0;
  width: 100%;
  height: 100%;
  border: 0;
}

.qcard img, .still {
  display: block;
  max-width: 100%;
  height: auto;
  margin: .7rem auto;
  background: #fff;
  border-radius: 10px;
  border: 1px solid var(--line);
}
.still.small { max-width: 220px; }

.letters {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: .5rem;
  margin: .6rem 0;
}
.letters figure {
  margin: 0;
  text-align: center;
  background: #fff;
  border: 1px solid var(--line);
  border-radius: 10px;
  padding: .4rem;
}
.letters img { width: 100%; height: auto; }
.letters figcaption { font-weight: 800; color: var(--navy); font-size: .9rem; }

.cards { display: grid; gap: .8rem; }
.flash {
  min-height: 140px;
  cursor: pointer;
  display: flex;
  flex-direction: column;
  justify-content: center;
}
.flash .term { font-weight: 800; font-size: 1.15rem; color: var(--navy); }
.flash .def { display: none; margin: 0; }
.flash.open .term { font-size: .85rem; color: var(--teal); margin-bottom: .4rem; }
.flash.open .def { display: block; }

.progress {
  color: var(--soft);
  font-size: .88rem;
  font-weight: 700;
  margin-bottom: .5rem;
}
.bar {
  height: 8px;
  background: #e8e2d6;
  border-radius: 99px;
  overflow: hidden;
  margin-bottom: 1rem;
}
.bar > span { display: block; height: 100%; background: var(--teal); width: 0; }

.choices { display: grid; gap: .45rem; }
.choice {
  text-align: left;
  border: 1px solid var(--line);
  background: #fff;
  border-radius: 10px;
  padding: .7rem .8rem;
  cursor: pointer;
  font-size: 1rem;
}
.choice.on { border-color: var(--teal); background: #e8f4f4; }
.choice.good { border-color: var(--good); background: var(--good-bg); }
.choice.bad { border-color: var(--bad); background: var(--bad-bg); }

.explain {
  margin-top: .8rem;
  padding: .75rem .85rem;
  border-radius: 10px;
  background: #eef6f6;
}
.explain.wrong { background: var(--bad-bg); }
.score {
  font-size: 2rem;
  font-weight: 800;
  color: var(--navy);
  margin: .2rem 0 .6rem;
}

footer {
  text-align: center;
  color: var(--soft);
  font-size: .8rem;
  padding: 1.5rem 1rem 2rem;
}