:root {
  --bg: #0f172a;
  --bg-card: #1e293b;
  --bg-input: #0b1220;
  --border: #334155;
  --text: #e2e8f0;
  --text-dim: #94a3b8;
  --accent: #38bdf8;
  --accent-dark: #0284c7;
  --green: #4ade80;
  --red: #f87171;
  --amber: #fbbf24;
  --radius: 12px;
}

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

body {
  font-family: "Segoe UI", system-ui, -apple-system, sans-serif;
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

/* ---------- Top bar ---------- */
.topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 0.5rem;
  padding: 0.9rem 1.5rem;
  background: var(--bg-card);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 10;
}

.brand { font-size: 1.25rem; font-weight: 700; letter-spacing: 0.5px; }

.tabs { display: flex; gap: 0.5rem; }

.tab-btn {
  background: transparent;
  border: 1px solid var(--border);
  color: var(--text-dim);
  padding: 0.45rem 1.1rem;
  border-radius: 999px;
  cursor: pointer;
  font-size: 0.95rem;
  transition: all 0.15s;
}
.tab-btn:hover { color: var(--text); border-color: var(--accent); }
.tab-btn.active { background: var(--accent); border-color: var(--accent); color: #082f49; font-weight: 600; }

/* ---------- Layout ---------- */
main { flex: 1; width: 100%; max-width: 760px; margin: 0 auto; padding: 1.5rem 1rem 3rem; }

.tab-panel { display: none; }
.tab-panel.active { display: block; }

.screen { display: none; }
.screen.active { display: block; }

h1 { font-size: 1.6rem; margin-bottom: 1rem; }
h2 { font-size: 1.1rem; margin-bottom: 0.6rem; color: var(--accent); }

.hint { color: var(--text-dim); font-size: 0.9rem; margin-bottom: 0.8rem; }

kbd {
  background: var(--bg-input);
  border: 1px solid var(--border);
  border-radius: 4px;
  padding: 0 0.35em;
  font-family: monospace;
}

.card {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.1rem 1.25rem;
  margin-bottom: 1rem;
}

/* ---------- Buttons ---------- */
button { font-family: inherit; }

.primary {
  background: var(--accent);
  color: #082f49;
  border: none;
  border-radius: var(--radius);
  padding: 0.65rem 1.4rem;
  font-size: 1rem;
  font-weight: 700;
  cursor: pointer;
  transition: background 0.15s, transform 0.05s;
}
.primary:hover { background: #7dd3fc; }
.primary:active { transform: scale(0.98); }

.secondary {
  background: transparent;
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 0.6rem 1.2rem;
  font-size: 0.95rem;
  cursor: pointer;
  transition: border-color 0.15s;
}
.secondary:hover { border-color: var(--accent); }

.ghost {
  background: transparent;
  color: var(--text-dim);
  border: none;
  padding: 0.6rem 1rem;
  cursor: pointer;
  font-size: 0.9rem;
}
.ghost:hover { color: var(--red); }

.big { width: 100%; padding: 0.9rem; font-size: 1.1rem; margin-top: 0.25rem; }

.error { color: var(--red); margin-top: 0.6rem; font-size: 0.9rem; }

/* ---------- Topic rows ---------- */
.topic-row {
  display: flex;
  align-items: center;
  gap: 0.85rem;
  padding: 0.65rem 0;
  border-bottom: 1px solid var(--border);
}
.topic-row:last-child { border-bottom: none; }

.topic-name { flex: 1; font-weight: 600; }
.op-symbol { color: var(--accent); font-weight: 700; margin-left: 0.2rem; }

.topic-row select {
  background: var(--bg-input);
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.45rem 0.6rem;
  font-size: 0.9rem;
  max-width: 230px;
}
.topic-row.disabled .topic-name,
.topic-row.disabled select { opacity: 0.4; pointer-events: none; }

/* Toggle switch */
.switch { position: relative; display: inline-block; width: 46px; height: 26px; flex-shrink: 0; }
.switch input { opacity: 0; width: 0; height: 0; }
.slider {
  position: absolute; inset: 0; cursor: pointer;
  background: var(--border); border-radius: 999px; transition: 0.2s;
}
.slider::before {
  content: ""; position: absolute; height: 20px; width: 20px;
  left: 3px; bottom: 3px; background: var(--text);
  border-radius: 50%; transition: 0.2s;
}
.switch input:checked + .slider { background: var(--accent); }
.switch input:checked + .slider::before { transform: translateX(20px); background: #082f49; }

/* ---------- Mode toggle ---------- */
.mode-toggle { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; }

.mode-btn {
  display: flex; flex-direction: column; align-items: center; gap: 0.3rem;
  background: var(--bg-input);
  border: 2px solid var(--border);
  border-radius: var(--radius);
  color: var(--text);
  padding: 0.9rem;
  font-size: 1.05rem;
  font-weight: 600;
  cursor: pointer;
  transition: border-color 0.15s;
}
.mode-btn small { color: var(--text-dim); font-weight: 400; font-size: 0.8rem; }
.mode-btn:hover { border-color: var(--accent); }
.mode-btn.active { border-color: var(--accent); background: rgba(56, 189, 248, 0.1); }

/* ---------- Session options ---------- */
.session-options { display: flex; flex-wrap: wrap; gap: 0.5rem 1.2rem; }
.session-options label {
  display: flex; align-items: center; gap: 0.4rem;
  cursor: pointer; font-size: 0.95rem;
}
.session-options input { accent-color: var(--accent); }

/* ---------- Quiz ---------- */
.quiz-status {
  display: flex; justify-content: space-between; flex-wrap: wrap; gap: 0.5rem;
  color: var(--text-dim); font-size: 0.95rem; margin-bottom: 1rem;
}
#quiz-timer { color: var(--amber); font-weight: 700; }
#quiz-score { color: var(--green); }

.problem-box {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  min-height: 150px;
  display: flex; flex-direction: column;
  align-items: center; justify-content: center;
  margin-bottom: 1rem;
  padding: 1rem;
}

.problem-text {
  font-size: clamp(2.2rem, 8vw, 3.4rem);
  font-weight: 700;
  letter-spacing: 2px;
  font-variant-numeric: tabular-nums;
}

.audio-controls { display: flex; flex-direction: column; align-items: center; gap: 0.7rem; }
.speaker-icon { font-size: 3rem; animation: pulse 1.6s infinite; }
@keyframes pulse { 0%, 100% { opacity: 1; } 50% { opacity: 0.45; } }

#answer-form { display: flex; gap: 0.6rem; margin-bottom: 0.8rem; }

#answer-input {
  flex: 1;
  background: var(--bg-input);
  border: 2px solid var(--border);
  border-radius: var(--radius);
  color: var(--text);
  font-size: 1.5rem;
  padding: 0.6rem 1rem;
  text-align: center;
  font-variant-numeric: tabular-nums;
}
#answer-input:focus { outline: none; border-color: var(--accent); }

.feedback {
  min-height: 2rem;
  text-align: center;
  font-size: 1.1rem;
  font-weight: 600;
  margin-bottom: 0.8rem;
}
.feedback.good { color: var(--green); }
.feedback.bad { color: var(--red); }

.quiz-actions { display: flex; justify-content: space-between; }

/* ---------- Results ---------- */
.results-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(130px, 1fr));
  gap: 1rem;
  text-align: center;
}
.stat-value { font-size: 2rem; font-weight: 700; color: var(--accent); }
.stat-label { color: var(--text-dim); font-size: 0.85rem; margin-top: 0.2rem; }

.missed-list { list-style: none; }
.missed-list li {
  padding: 0.5rem 0;
  border-bottom: 1px solid var(--border);
  font-size: 1.05rem;
  font-variant-numeric: tabular-nums;
}
.missed-list li:last-child { border-bottom: none; }
.missed-list .your-answer { color: var(--red); text-decoration: line-through; margin-left: 0.5rem; }
.missed-list .right-answer { color: var(--green); font-weight: 700; margin-left: 0.5rem; }

.results-actions { display: flex; flex-direction: column; gap: 0.6rem; margin-top: 1rem; }

/* ---------- Techniques ---------- */
.tech {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  margin-bottom: 0.8rem;
  overflow: hidden;
}
.tech summary {
  cursor: pointer;
  padding: 1rem 1.25rem;
  font-size: 1.15rem;
  font-weight: 700;
  list-style: none;
  user-select: none;
}
.tech summary::-webkit-details-marker { display: none; }
.tech summary::after { content: "▾"; float: right; color: var(--text-dim); transition: transform 0.2s; }
.tech[open] summary::after { transform: rotate(180deg); }

.tech-body { padding: 0 1.25rem 1.25rem; }
.tech-body h3 { margin: 1rem 0 0.35rem; font-size: 1rem; color: var(--accent); }
.tech-body p, .tech-body li { color: var(--text); font-size: 0.95rem; line-height: 1.55; }
.tech-body ul { padding-left: 1.2rem; margin-top: 0.4rem; }
.tech-body li { margin-bottom: 0.4rem; }

.example {
  background: var(--bg-input);
  border-left: 3px solid var(--accent);
  border-radius: 0 8px 8px 0;
  padding: 0.5rem 0.8rem;
  margin: 0.4rem 0;
  font-variant-numeric: tabular-nums;
}

/* ---------- Footer ---------- */
footer {
  text-align: center;
  color: var(--text-dim);
  font-size: 0.8rem;
  padding: 1.2rem;
  border-top: 1px solid var(--border);
}

@media (max-width: 520px) {
  .topic-row { flex-wrap: wrap; }
  .topic-row select { max-width: 100%; width: 100%; }
  .mode-toggle { grid-template-columns: 1fr; }
}