* { box-sizing: border-box; margin: 0; padding: 0; }

:root {
  --bg: #f4f6fb;
  --card: #ffffff;
  --ink: #1e2533;
  --muted: #66708a;
  --accent: #3556c7;
  --accent-soft: #e3e9fb;
  --green: #1f9d61;
  --green-soft: #e0f5ea;
  --red: #d0453e;
  --red-soft: #fbe6e5;
  --amber: #b97b0e;
  --amber-soft: #fdf1da;
  --radius: 12px;
  --shadow: 0 1px 3px rgba(20, 30, 60, .08), 0 6px 18px rgba(20, 30, 60, .06);
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  background: var(--bg);
  color: var(--ink);
  line-height: 1.55;
}

.wrap { max-width: 980px; margin: 0 auto; padding: 24px 16px 64px; }

header.site {
  background: var(--accent);
  color: #fff;
  padding: 22px 16px;
}
header.site .wrap { padding: 0; max-width: 980px; margin: 0 auto; }
header.site h1 { font-size: 1.5rem; font-weight: 700; }
header.site h1 a { color: inherit; text-decoration: none; }
header.site p { opacity: .85; font-size: .95rem; margin-top: 2px; }
header.site nav { margin-top: 10px; display: flex; gap: 14px; }
header.site nav a { color: #fff; opacity: .9; text-decoration: none; font-size: .9rem; border-bottom: 2px solid transparent; padding-bottom: 2px; }
header.site nav a:hover, header.site nav a.active { opacity: 1; border-bottom-color: #fff; }

.banner {
  margin: 20px 0 0;
  padding: 14px 18px;
  border-radius: var(--radius);
  background: var(--green-soft);
  color: #14532d;
  border: 1px solid #bde5cf;
}
.banner strong a { color: #14532d; }
.banner.info { background: var(--amber-soft); border-color: #f0dcae; color: #6b4a08; }
.banner.schedule { background: var(--accent-soft); border-color: #c9d5f5; color: #23386e; }

h2.level { margin: 34px 0 14px; font-size: 1.15rem; color: var(--muted); text-transform: uppercase; letter-spacing: .06em; }

/* collapsible category sections */
details.level-group { margin-top: 16px; }
details.level-group summary {
  cursor: pointer; list-style: none;
  display: flex; align-items: center; gap: 12px;
  background: var(--card); border-radius: var(--radius); box-shadow: var(--shadow);
  padding: 15px 20px; font-weight: 600; font-size: 1.05rem;
  user-select: none;
}
details.level-group summary::-webkit-details-marker { display: none; }
details.level-group summary::before {
  content: '▸'; color: var(--accent); font-size: 1rem;
  transition: transform .15s ease;
}
details.level-group[open] summary::before { transform: rotate(90deg); }
details.level-group summary .lvl-count {
  margin-left: auto; font-weight: 400; font-size: .85rem; color: var(--muted);
  background: var(--accent-soft); border-radius: 999px; padding: 3px 12px;
}
details.level-group summary .lvl-count.done {
  margin-left: auto; background: var(--green-soft); color: var(--green);
}
details.level-group summary .lvl-count.done + .lvl-count { margin-left: 0; }
details.level-group summary:hover { outline: 2px solid var(--accent-soft); }
details.level-group .grid { margin-top: 14px; padding-left: 4px; }

/* teacher login */
.login-box { max-width: 420px; margin: 40px auto 0; text-align: center; }
.login-box h2 { margin-bottom: 8px; }
.login-hint { color: var(--muted); font-size: .92rem; margin-bottom: 10px; }
.login-error { color: var(--red); font-weight: 600; margin-bottom: 8px; }

.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(215px, 1fr)); gap: 14px; }

.topic-card {
  display: block;
  background: var(--card);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 16px;
  text-decoration: none;
  color: var(--ink);
  border: 2px solid transparent;
  transition: transform .12s ease, border-color .12s ease;
}
.topic-card:hover { transform: translateY(-2px); border-color: var(--accent); }
.topic-card h3 { font-size: 1.02rem; margin-bottom: 4px; }
.topic-card .meta { font-size: .82rem; color: var(--muted); }
.topic-card.chosen { border-color: var(--green); background: var(--green-soft); }
.topic-card .badge { display: inline-block; margin-top: 8px; font-size: .75rem; background: var(--green); color: #fff; border-radius: 999px; padding: 2px 10px; }
.topic-card.covered { opacity: .68; }
.topic-card.covered:hover { opacity: 1; }
.topic-card .badge.done { background: transparent; border: 1px solid var(--green); color: var(--green); }

/* topic page */
.topic-head { margin: 26px 0 18px; display: flex; flex-wrap: wrap; align-items: center; gap: 12px; justify-content: space-between; }
.topic-head h2 { font-size: 1.6rem; }
.topic-head .crumbs { font-size: .85rem; color: var(--muted); margin-bottom: 4px; }
.topic-head .crumbs a { color: var(--accent); text-decoration: none; }

.btn {
  display: inline-block; border: 0; cursor: pointer; font: inherit;
  background: var(--accent); color: #fff; border-radius: 8px;
  padding: 9px 18px; font-size: .95rem; text-decoration: none;
}
.btn:hover { filter: brightness(1.08); }
.btn.secondary { background: var(--accent-soft); color: var(--accent); }
.btn.green { background: var(--green); }
.btn:disabled { opacity: .55; cursor: default; }

.tabs { display: flex; gap: 6px; flex-wrap: wrap; margin-bottom: 18px; }
.tabs button {
  font: inherit; font-size: .92rem; cursor: pointer;
  border: 1px solid #d6dcef; background: var(--card); color: var(--muted);
  border-radius: 999px; padding: 7px 18px;
}
.tabs button.active { background: var(--accent); border-color: var(--accent); color: #fff; }

.panel { background: var(--card); border-radius: var(--radius); box-shadow: var(--shadow); padding: 22px; }

table.words { width: 100%; border-collapse: collapse; }
table.words th, table.words td { text-align: left; padding: 10px 12px; border-bottom: 1px solid #edf0f7; vertical-align: top; }
table.words th { font-size: .8rem; text-transform: uppercase; letter-spacing: .05em; color: var(--muted); }
table.words td.en { font-weight: 600; white-space: nowrap; }
table.words td.ko { color: var(--accent); }
table.words td.ex { color: var(--muted); font-style: italic; }

/* flashcards */
.flash-area { text-align: center; }
.flashcard {
  margin: 0 auto 18px; max-width: 430px; min-height: 210px;
  background: var(--accent-soft); border: 2px solid var(--accent);
  border-radius: var(--radius); cursor: pointer;
  display: flex; flex-direction: column; align-items: center; justify-content: center;
  padding: 24px; gap: 10px;
}
.flashcard .word { font-size: 1.7rem; font-weight: 700; }
.flashcard .hint { font-size: .8rem; color: var(--muted); }
.flashcard .example { font-style: italic; color: var(--muted); }
.flash-controls { display: flex; gap: 10px; justify-content: center; flex-wrap: wrap; }
.flash-progress { margin-top: 12px; color: var(--muted); font-size: .88rem; }

/* quiz */
.quiz-question { font-size: 1.25rem; margin-bottom: 6px; }
.quiz-sub { color: var(--muted); margin-bottom: 16px; }
.quiz-options { display: grid; gap: 10px; max-width: 520px; }
.quiz-options button {
  font: inherit; text-align: left; cursor: pointer;
  background: var(--bg); border: 2px solid #d6dcef; border-radius: 10px; padding: 12px 16px;
}
.quiz-options button:hover:not(:disabled) { border-color: var(--accent); }
.quiz-options button.correct { background: var(--green-soft); border-color: var(--green); }
.quiz-options button.wrong { background: var(--red-soft); border-color: var(--red); }
.quiz-feedback { margin-top: 14px; font-weight: 600; min-height: 1.5em; }
.quiz-feedback.good { color: var(--green); }
.quiz-feedback.bad { color: var(--red); }
.quiz-score { color: var(--muted); font-size: .9rem; margin-top: 8px; }

.spell-row { display: flex; gap: 10px; align-items: center; flex-wrap: wrap; margin: 12px 0; }
.spell-input {
  font: inherit; font-size: 1.1rem; padding: 10px 14px; flex: 1 1 220px; max-width: 380px;
  border: 2px solid #d6dcef; border-radius: 10px;
}
.spell-input:focus { outline: none; border-color: var(--accent); }

.result-box { text-align: center; padding: 20px 0; }
.result-box .big { font-size: 2.4rem; font-weight: 700; color: var(--accent); }

/* teacher page */
.score-table { width: 100%; border-collapse: collapse; }
.score-table th, .score-table td { padding: 9px 12px; border-bottom: 1px solid #edf0f7; text-align: left; font-size: .93rem; }
.score-table th { color: var(--muted); font-size: .8rem; text-transform: uppercase; letter-spacing: .05em; }
.pill { display: inline-block; border-radius: 999px; padding: 2px 10px; font-size: .8rem; font-weight: 600; }
.pill.good { background: var(--green-soft); color: var(--green); }
.pill.mid { background: var(--amber-soft); color: var(--amber); }
.pill.low { background: var(--red-soft); color: var(--red); }

footer.site { margin-top: 40px; text-align: center; color: var(--muted); font-size: .82rem; }
footer.site a { color: var(--muted); }

@media (max-width: 640px) {
  .wrap { padding: 14px 12px 48px; }
  .panel { padding: 16px 14px; }
  .topic-head { flex-direction: column; align-items: stretch; margin: 18px 0 14px; }
  .topic-head h2 { font-size: 1.35rem; }
  #choose-btn { width: 100%; padding: 12px; }

  /* tabs become an even, thumb-sized row */
  .tabs { gap: 8px; }
  .tabs button { flex: 1 1 40%; padding: 11px 6px; text-align: center; }

  /* study table becomes stacked word cards */
  table.words thead { display: none; }
  table.words, table.words tbody, table.words tr, table.words td { display: block; width: 100%; }
  table.words tr { padding: 10px 2px; border-bottom: 1px solid #edf0f7; }
  table.words tr:last-child { border-bottom: none; }
  table.words td { border: none; padding: 1px 0; }
  table.words td.en { white-space: normal; font-size: 1.05rem; }

  /* flashcards */
  .flashcard { min-height: 180px; padding: 18px; }
  .flashcard .word { font-size: 1.35rem; }
  .flash-controls { display: grid; grid-template-columns: 1fr 1fr; gap: 8px; }
  .flash-controls #flash-mode { grid-column: 1 / -1; order: 2; }
  .flash-controls .btn { padding: 12px 8px; }

  /* quiz + spelling */
  .quiz-question { font-size: 1.1rem; }
  .quiz-options button { padding: 13px 14px; }
  .spell-input { max-width: none; }
  .spell-row .btn { flex: 1 1 100%; padding: 12px; }
}