:root {
  color-scheme: dark;
  --bg: #0b1020;
  --panel: rgba(20, 27, 48, 0.88);
  --panel-2: rgba(30, 39, 67, 0.92);
  --text: #eef2ff;
  --muted: #9aa4c7;
  --line: rgba(255, 255, 255, 0.08);
  --accent: #7c9cff;
  --accent-2: #9d7bff;
  --success: #2fb37b;
  --danger: #d45a73;
  --shadow: 0 18px 50px rgba(0, 0, 0, 0.35);
}

* { box-sizing: border-box; }
body {
  margin: 0;
  min-height: 100vh;
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  background:
    radial-gradient(circle at top left, rgba(124, 156, 255, 0.18), transparent 28%),
    radial-gradient(circle at top right, rgba(157, 123, 255, 0.16), transparent 30%),
    linear-gradient(180deg, #0b1020 0%, #0f1530 100%);
  color: var(--text);
}

button, select {
  font: inherit;
}

.app-shell {
  width: min(980px, calc(100% - 32px));
  margin: 0 auto;
  padding: 28px 0 48px;
}

.topbar {
  display: flex;
  justify-content: space-between;
  gap: 16px;
  align-items: start;
  margin-bottom: 20px;
}

.eyebrow {
  margin: 0 0 6px;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: 0.12em;
  font-size: 0.75rem;
  font-weight: 700;
}

h1 {
  margin: 0;
  font-size: clamp(1.8rem, 3vw, 3rem);
}

.stats-grid,
.controls-card,
.card-section,
.empty-state {
  background: var(--panel);
  backdrop-filter: blur(18px);
  border: 1px solid var(--line);
  box-shadow: var(--shadow);
}

.stats-grid {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 14px;
  margin-bottom: 18px;
}

.stat-card {
  padding: 18px;
  border-radius: 18px;
  background: rgba(255,255,255,0.03);
}

.stat-card span {
  display: block;
  color: var(--muted);
  margin-bottom: 10px;
  font-size: 0.92rem;
}

.stat-card strong {
  font-size: 1.65rem;
}

.controls-card {
  padding: 18px;
  border-radius: 22px;
  margin-bottom: 20px;
}

.control-row {
  display: flex;
  justify-content: space-between;
  gap: 16px;
  flex-wrap: wrap;
  margin-bottom: 18px;
}

.switch-row {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  color: var(--muted);
}

.checkbox-row input {
  width: 18px;
  height: 18px;
}

select {
  background: var(--panel-2);
  color: var(--text);
  border: 1px solid var(--line);
  border-radius: 12px;
  padding: 10px 12px;
}

.progress-label-row {
  display: flex;
  justify-content: space-between;
  color: var(--muted);
  margin-bottom: 8px;
  font-size: 0.94rem;
}

.progress-line {
  width: 100%;
  height: 12px;
  background: rgba(255,255,255,0.06);
  border-radius: 999px;
  overflow: hidden;
}

.progress-line-bar {
  width: 0%;
  height: 100%;
  border-radius: inherit;
  background: linear-gradient(90deg, var(--accent), var(--accent-2));
  transition: width 180ms ease;
}

.card-stage {
  min-height: 520px;
}

.card-section,
.empty-state {
  border-radius: 28px;
  padding: 22px;
}

.card-meta-row {
  display: flex;
  justify-content: space-between;
  gap: 10px;
  color: var(--muted);
  margin-bottom: 16px;
}

.flashcard {
  width: 100%;
  min-height: 340px;
  background: transparent;
  border: 0;
  padding: 0;
  perspective: 1400px;
  cursor: pointer;
}

.flashcard-inner {
  position: relative;
  width: 100%;
  min-height: 340px;
  transition: transform 0.55s ease;
  transform-style: preserve-3d;
}

.flashcard.is-flipped .flashcard-inner {
  transform: rotateY(180deg);
}

.flashcard.no-flip-transition .flashcard-inner {
  transition: none;
}

.flashcard-face {
  position: absolute;
  inset: 0;
  background: linear-gradient(180deg, rgba(255,255,255,0.04), rgba(255,255,255,0.02));
  border: 1px solid var(--line);
  border-radius: 26px;
  padding: 30px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  backface-visibility: hidden;
}

.flashcard-front h2,
.flashcard-back h2 {
  margin: 0;
  text-align: center;
  font-size: clamp(1.8rem, 4vw, 3rem);
  line-height: 1.2;
}

.flashcard-back {
  transform: rotateY(180deg);
  background: linear-gradient(180deg, rgba(124, 156, 255, 0.12), rgba(157, 123, 255, 0.12));
}

.face-label {
  margin: 0 0 14px;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.12em;
  font-size: 0.78rem;
  font-weight: 700;
}

.flip-hint {
  text-align: center;
  color: var(--muted);
  margin: 16px 0 0;
}

.rating-buttons {
  margin-top: 18px;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 12px;
}

button[class$="button"],
.ghost-button {
  border: 0;
  border-radius: 14px;
  padding: 12px 16px;
  cursor: pointer;
  transition: transform 120ms ease, opacity 120ms ease, background 120ms ease;
}

button:hover { transform: translateY(-1px); }
button:active { transform: translateY(0); }

.primary-button { background: linear-gradient(90deg, var(--accent), var(--accent-2)); color: white; }
.secondary-button { background: rgba(255,255,255,0.12); color: var(--text); }
.success-button { background: var(--success); color: white; }
.danger-button { background: var(--danger); color: white; }
.ghost-button { background: transparent; color: var(--muted); border: 1px solid var(--line); }

.empty-state {
  text-align: center;
}

.empty-state h2 {
  margin-top: 8px;
  margin-bottom: 12px;
  font-size: 2rem;
}

.empty-state p {
  color: var(--muted);
  margin-bottom: 22px;
}

.hidden { display: none !important; }

@media (max-width: 720px) {
  .app-shell { width: min(100% - 20px, 980px); }
  .stats-grid { grid-template-columns: repeat(2, 1fr); }
  .rating-buttons { grid-template-columns: repeat(2, 1fr); }
  .topbar { flex-direction: column; }
  .flashcard, .flashcard-inner { min-height: 280px; }
}