:root {
  --bg: #14171c;
  --panel: #1d2128;
  --panel-hover: #262b34;
  --accent: #5b9dff;
  --accent-soft: #2a3a52;
  --text-main: #eef1f6;
  --text-dim: #98a2b3;
  --border: #2c313a;
  --success: #6fcf97;
  --radius: 14px;
  --shadow: 0 8px 24px rgba(0,0,0,0.35);
}

* {
  box-sizing: border-box;
}

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text-main);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  min-height: 100vh;
}

.app-header {
  text-align: center;
  padding: 36px 20px 18px;
}

.app-header h1 {
  margin: 0 0 8px;
  font-size: 2rem;
  letter-spacing: -0.01em;
}

.subtitle {
  margin: 0;
  color: var(--text-dim);
  font-size: 0.95rem;
}

#app {
  max-width: 1000px;
  margin: 0 auto;
  padding: 10px 20px 60px;
}

.screen.hidden {
  display: none;
}

/* ---------- Topic selection ---------- */

.topic-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(230px, 1fr));
  gap: 16px;
  margin-top: 20px;
}

.topic-card {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 22px 20px;
  cursor: pointer;
  transition: transform 0.12s ease, background 0.12s ease, border-color 0.12s ease;
  text-align: left;
}

.topic-card:hover {
  background: var(--panel-hover);
  border-color: var(--accent);
  transform: translateY(-2px);
}

.topic-card .topic-name {
  font-size: 1.08rem;
  font-weight: 600;
  margin-bottom: 6px;
  display: block;
}

.topic-card .topic-count {
  color: var(--text-dim);
  font-size: 0.88rem;
}

.topic-card.randomize-card {
  background: linear-gradient(135deg, var(--accent-soft), var(--panel));
  border-color: var(--accent);
  grid-column: 1 / -1;
}

.topic-card.randomize-card .topic-name {
  font-size: 1.25rem;
  color: var(--accent);
}

/* ---------- Study screen ---------- */

.study-toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  margin: 18px 0 8px;
  flex-wrap: wrap;
}

.study-info {
  display: flex;
  flex-direction: column;
  align-items: center;
  text-align: center;
  flex: 1;
}

.study-topic-name {
  font-weight: 600;
  font-size: 1.05rem;
}

.study-progress {
  color: var(--text-dim);
  font-size: 0.85rem;
  margin-top: 2px;
}

.btn {
  font-family: inherit;
  font-size: 0.92rem;
  border-radius: 10px;
  padding: 10px 16px;
  border: 1px solid var(--border);
  background: var(--panel);
  color: var(--text-main);
  cursor: pointer;
  transition: background 0.12s ease, border-color 0.12s ease;
}

.btn:hover {
  background: var(--panel-hover);
  border-color: var(--accent);
}

.btn-secondary {
  white-space: nowrap;
}

.btn-nav {
  flex: 1;
  max-width: 220px;
  font-size: 1rem;
  padding: 12px 16px;
}

.card-stage {
  display: flex;
  justify-content: center;
  margin: 22px 0;
}

.flashcard {
  width: 100%;
  max-width: 700px;
  min-height: 280px;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 36px 32px;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  outline: none;
}

.flashcard:focus-visible {
  border-color: var(--accent);
}

.card-face {
  width: 100%;
  text-align: center;
}

.card-label {
  text-transform: uppercase;
  letter-spacing: 0.08em;
  font-size: 0.72rem;
  color: var(--text-dim);
  margin-bottom: 10px;
}

.card-question {
  font-size: 1.3rem;
  line-height: 1.5;
  font-weight: 600;
}

.answer-block.hidden {
  display: none;
}

.divider {
  border: none;
  border-top: 1px solid var(--border);
  margin: 22px 0;
}

.card-answer {
  font-size: 1.08rem;
  line-height: 1.6;
  color: var(--success);
  text-align: left;
  white-space: pre-line;
}

.reveal-hint {
  margin-top: 22px;
  color: var(--text-dim);
  font-size: 0.82rem;
}

.flashcard.revealed .reveal-hint {
  display: none;
}

.nav-row {
  display: flex;
  justify-content: center;
  gap: 16px;
  margin-top: 8px;
}

.app-footer {
  text-align: center;
  color: var(--text-dim);
  font-size: 0.8rem;
  padding: 20px 0 40px;
}

@media (max-width: 480px) {
  .flashcard {
    padding: 26px 18px;
  }
  .card-question {
    font-size: 1.12rem;
  }
}