:root {
  --bg: #f4f7fb;
  --card: #ffffff;
  --border: #e2e8f0;
  --text: #1f2933;
  --muted: #64748b;
  --accent: #0ea5e9;
  --accent-dark: #0284c7;
  --accent-soft: #e0f2fe;
  --success: #22c55e;
  --error: #f43f5e;
  font-family: 'SUIT', system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  line-height: 1.5;
}

a {
  color: var(--accent-dark);
}

main {
  width: min(1100px, 92vw);
  margin: 0 auto 4rem;
  display: flex;
  flex-direction: column;
  gap: 1.5rem;
}

.app-header {
  width: min(1100px, 92vw);
  margin: 2rem auto 1rem;
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  gap: 1rem;
}

.app-header h1 {
  margin: 0.2rem 0 0.4rem;
  font-size: clamp(1.8rem, 3vw, 2.4rem);
}

.app-header .eyebrow {
  text-transform: uppercase;
  letter-spacing: 0.12em;
  font-size: 0.8rem;
  color: var(--accent-dark);
  margin: 0;
}

.subtitle {
  margin: 0;
  color: var(--muted);
}

.header-meta {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  font-weight: 600;
}

button {
  border: none;
  border-radius: 999px;
  padding: 0.55rem 1.2rem;
  font-weight: 600;
  font-family: inherit;
  cursor: pointer;
  transition: transform 0.15s ease, box-shadow 0.15s ease;
}

button:disabled {
  cursor: not-allowed;
  opacity: 0.6;
}

button:not(:disabled):active {
  transform: translateY(1px);
}

button.ghost {
  background: transparent;
  border: 1px solid var(--border);
}

button.primary {
  background: var(--accent);
  color: #fff;
}

.card {
  background: var(--card);
  border-radius: 20px;
  padding: 1.5rem;
  box-shadow: 0 20px 35px rgba(15, 23, 42, 0.08);
}

.control-panel {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 1.25rem;
  align-items: flex-end;
}

.control-group label,
.control-group .label {
  font-size: 0.9rem;
  font-weight: 600;
  display: block;
  margin-bottom: 0.35rem;
}

.control-group select,
.control-group input[type='search'] {
  width: 100%;
  border-radius: 12px;
  border: 1px solid var(--border);
  padding: 0.65rem 0.85rem;
  font-family: inherit;
  font-size: 1rem;
}

.mode-group {
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
  color: var(--muted);
  font-weight: 500;
}

.control-actions {
  display: flex;
  justify-content: flex-end;
}

.quiz {
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.stats {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(120px, 1fr));
  gap: 0.75rem;
}

.stats .label {
  display: block;
  font-size: 0.8rem;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.08em;
}

.stats strong {
  font-size: 1.4rem;
}

.question {
  padding: 1.25rem;
  border-radius: 16px;
  background: var(--accent-soft);
}

.question .prompt {
  margin: 0 0 0.5rem;
  font-weight: 600;
}

.question .body {
  margin: 0;
  font-size: 1.15rem;
  font-weight: 600;
}

.options {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 0.75rem;
}

.option-btn {
  border-radius: 16px;
  padding: 1rem;
  border: 1px solid var(--border);
  text-align: left;
  background: #fff;
  transition: border-color 0.2s ease, box-shadow 0.2s ease, background 0.2s ease;
}

.option-btn:hover:not(.locked) {
  border-color: var(--accent);
  box-shadow: 0 4px 20px rgba(14, 165, 233, 0.2);
}

.option-btn.correct {
  border-color: var(--success);
  background: rgba(34, 197, 94, 0.12);
}

.option-btn.incorrect {
  border-color: var(--error);
  background: rgba(244, 63, 94, 0.12);
}

.option-btn.locked {
  cursor: default;
}

.feedback {
  min-height: 1.5rem;
  font-weight: 600;
}

.feedback.correct {
  color: var(--success);
}

.feedback.incorrect {
  color: var(--error);
}

.reference-header {
  display: flex;
  flex-direction: column;
  gap: 0.3rem;
}

.reference-header h2 {
  margin: 0;
}

.reference-header .hint {
  color: var(--muted);
  font-size: 0.9rem;
}

.formula-list {
  list-style: none;
  padding: 0;
  margin: 1rem 0 0;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
  gap: 0.75rem;
}

.formula-item {
  border: 1px solid var(--border);
  border-radius: 16px;
  padding: 0.9rem;
  background: #fff;
}

.formula-item h3 {
  margin: 0 0 0.35rem;
  font-size: 1rem;
}

.formula-item p {
  margin: 0;
  color: var(--muted);
  font-size: 0.95rem;
}

.formula-item .category {
  display: inline-flex;
  align-items: center;
  gap: 0.25rem;
  font-size: 0.8rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--accent-dark);
  margin-bottom: 0.35rem;
}

footer {
  width: min(1100px, 92vw);
  margin: 0 auto 2rem;
  color: var(--muted);
  text-align: center;
}

@media (max-width: 640px) {
  .app-header {
    flex-direction: column;
  }

  .options {
    grid-template-columns: 1fr;
  }
}