:root {
  --bg-from: #f1f1f1;
  --bg-to: #f1f1f1;
  --panel: #ffffff;
  --panel-2: #e2e8f0;
  --panel-2-hover: #cbd5e1;
  --text: #0f172a;
  --muted: #475569;
  --accent: #0284c7;
  --accent-text: #ffffff;
  --correct: #16a34a;
  --correct-text: #ffffff;
  --wrong: #dc2626;
  --wrong-text: #ffffff;
  --status-error: #b91c1c;
  --status-success: #15803d;
  --code-bg: rgba(15, 23, 42, 0.08);
  --paste-bg: rgba(15, 23, 42, 0.04);
  --input-bg: #f8fafc;
  --border: #cbd5e1;
  --card-front-from: #3b82f6;
  --card-front-to: #8b5cf6;
  --card-back-from: #10b981;
  --card-back-to: #059669;
  --card-text: #ffffff;
  --shadow: 0 10px 30px rgba(15, 23, 42, 0.12);
  --radius: 14px;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg-from: #0f172a;
    --bg-to: #1e1b4b;
    --panel: #1e293b;
    --panel-2: #334155;
    --panel-2-hover: #475569;
    --text: #f1f5f9;
    --muted: #94a3b8;
    --accent: #38bdf8;
    --accent-text: #0f172a;
    --correct: #22c55e;
    --correct-text: #052e16;
    --wrong: #ef4444;
    --wrong-text: #450a0a;
    --status-error: #fca5a5;
    --status-success: #86efac;
    --code-bg: rgba(255, 255, 255, 0.08);
    --paste-bg: rgba(255, 255, 255, 0.04);
    --input-bg: #0f172a;
    --border: #475569;
    --card-front-from: #1e3a8a;
    --card-front-to: #3b0764;
    --card-back-from: #065f46;
    --card-back-to: #064e3b;
    --card-text: #f1f5f9;
    --shadow: 0 10px 30px rgba(0, 0, 0, 0.35);
  }
}

:root[data-theme="light"] {
  --bg-from: #f1f1f1;
  --bg-to: #f1f1f1;
  --panel: #ffffff;
  --panel-2: #e2e8f0;
  --panel-2-hover: #cbd5e1;
  --text: #0f172a;
  --muted: #475569;
  --accent: #0284c7;
  --accent-text: #ffffff;
  --correct: #16a34a;
  --correct-text: #ffffff;
  --wrong: #dc2626;
  --wrong-text: #ffffff;
  --status-error: #b91c1c;
  --status-success: #15803d;
  --code-bg: rgba(15, 23, 42, 0.08);
  --paste-bg: rgba(15, 23, 42, 0.04);
  --input-bg: #f8fafc;
  --border: #cbd5e1;
  --card-front-from: #3b82f6;
  --card-front-to: #8b5cf6;
  --card-back-from: #10b981;
  --card-back-to: #059669;
  --card-text: #ffffff;
  --shadow: 0 10px 30px rgba(15, 23, 42, 0.12);
}

:root[data-theme="dark"] {
  --bg-from: #0f172a;
  --bg-to: #1e1b4b;
  --panel: #1e293b;
  --panel-2: #334155;
  --panel-2-hover: #475569;
  --text: #f1f5f9;
  --muted: #94a3b8;
  --accent: #38bdf8;
  --accent-text: #0f172a;
  --correct: #22c55e;
  --correct-text: #052e16;
  --wrong: #ef4444;
  --wrong-text: #450a0a;
  --status-error: #fca5a5;
  --status-success: #86efac;
  --code-bg: rgba(255, 255, 255, 0.08);
  --paste-bg: rgba(255, 255, 255, 0.04);
  --input-bg: #0f172a;
  --border: #475569;
  --card-front-from: #1e3a8a;
  --card-front-to: #3b0764;
  --card-back-from: #065f46;
  --card-back-to: #064e3b;
  --card-text: #f1f5f9;
  --shadow: 0 10px 30px rgba(0, 0, 0, 0.35);
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: linear-gradient(135deg, var(--bg-from) 0%, var(--bg-to) 100%);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  min-height: 100vh;
  transition: background 0.3s ease, color 0.3s ease;
}

.app {
  max-width: 720px;
  margin: 0 auto;
  padding: 32px 20px 64px;
}

.app-header {
  display: flex;
  align-items: center;
  justify-content: center;
  position: relative;
  margin-bottom: 24px;
}

.app-header h1 {
  font-size: 1.75rem;
  margin: 0;
  text-align: center;
  letter-spacing: 0.5px;
}

.theme-picker {
  position: absolute;
  right: 0;
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-size: 0.85rem;
  color: var(--muted);
}
.theme-picker select {
  background: var(--panel-2);
  color: var(--text);
  border: 1px solid var(--border);
  padding: 4px 8px;
  border-radius: 8px;
  font: inherit;
  cursor: pointer;
}

.panel {
  background: var(--panel);
  border-radius: var(--radius);
  padding: 24px;
  box-shadow: var(--shadow);
  margin-bottom: 20px;
}

.panel.hidden { display: none; }

.hint {
  color: var(--muted);
  font-size: 0.9rem;
  margin-top: 0;
}
.hint code {
  background: var(--code-bg);
  padding: 1px 6px;
  border-radius: 4px;
}

.row {
  display: flex;
  flex-wrap: wrap;
  gap: 12px;
  align-items: center;
  margin-bottom: 16px;
}

.file-label {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  background: var(--panel-2);
  color: var(--text);
  padding: 10px 16px;
  border-radius: 10px;
  cursor: pointer;
  transition: background 0.15s ease;
}
.file-label:hover { background: var(--panel-2-hover); }
.file-label input[type="file"] {
  position: absolute;
  width: 1px; height: 1px;
  opacity: 0;
  pointer-events: none;
}

button {
  font: inherit;
  font-weight: 600;
  background: var(--accent);
  color: var(--accent-text);
  border: none;
  padding: 10px 18px;
  border-radius: 10px;
  cursor: pointer;
  transition: transform 0.05s ease, opacity 0.15s ease, background 0.15s ease;
}
button:hover:not(:disabled) { transform: translateY(-1px); }
button:disabled { opacity: 0.45; cursor: not-allowed; }
button.secondary {
  background: var(--panel-2);
  color: var(--text);
}
button.secondary:hover:not(:disabled) { background: var(--panel-2-hover); }
button.correct { background: var(--correct); color: var(--correct-text); }
button.wrong { background: var(--wrong); color: var(--wrong-text); }

.paste {
  margin: 12px 0 16px;
  background: var(--paste-bg);
  padding: 12px 14px;
  border-radius: 10px;
}
.paste summary {
  cursor: pointer;
  color: var(--muted);
  margin-bottom: 8px;
}
.paste textarea {
  width: 100%;
  background: var(--input-bg);
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 10px;
  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
  font-size: 0.9rem;
  resize: vertical;
  margin-bottom: 8px;
}

.options {
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 12px 16px;
  margin: 0;
}
.options legend {
  padding: 0 6px;
  color: var(--muted);
}
.options label {
  display: inline-flex;
  align-items: center;
  gap: 8px;
}
.options select {
  background: var(--panel-2);
  color: var(--text);
  border: 1px solid var(--border);
  padding: 6px 10px;
  border-radius: 8px;
  font: inherit;
}

.status {
  margin: 14px 0 0;
  min-height: 1.2em;
  color: var(--muted);
}
.status.error { color: var(--status-error); }
.status.success { color: var(--status-success); }

.stats {
  display: flex;
  gap: 18px;
  justify-content: center;
  flex-wrap: wrap;
  color: var(--muted);
  margin-bottom: 18px;
  font-size: 0.95rem;
}
.stats strong { color: var(--text); }

.card {
  perspective: 1200px;
  cursor: pointer;
  user-select: none;
  outline: none;
  margin-bottom: 18px;
}
.card:focus-visible .card-inner {
  box-shadow: 0 0 0 3px var(--accent), var(--shadow);
}

.card-inner {
  position: relative;
  width: 100%;
  min-height: 260px;
  border-radius: var(--radius);
  transition: transform 0.55s cubic-bezier(0.4, 0.0, 0.2, 1);
  transform-style: preserve-3d;
  box-shadow: var(--shadow);
}

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

.card-face {
  position: absolute;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 28px;
  border-radius: var(--radius);
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  font-size: clamp(1.1rem, 3vw, 1.75rem);
  text-align: center;
  font-weight: 600;
  line-height: 1.4;
  overflow-wrap: anywhere;
  word-break: break-word;
  hyphens: auto;
  color: var(--card-text);
}
.card-face span { max-height: 100%; overflow-y: auto; }
.card-face.long { font-size: clamp(0.95rem, 2.2vw, 1.2rem); }
.card-front {
  background: linear-gradient(135deg, var(--card-front-from), var(--card-front-to));
}
.card-back {
  background: linear-gradient(135deg, var(--card-back-from), var(--card-back-to));
  transform: rotateY(180deg);
}

.card-hint {
  text-align: center;
  color: var(--muted);
  font-size: 0.85rem;
  margin: 10px 0 0;
}

.actions {
  display: flex;
  gap: 12px;
  justify-content: center;
  flex-wrap: wrap;
}
.actions.secondary-row {
  margin-top: 14px;
}

@media (max-width: 560px) {
  .theme-picker { position: static; margin-top: 8px; }
  .app-header { flex-direction: column; gap: 8px; }
}

@media (max-width: 480px) {
  .app { padding: 20px 14px 40px; }
  .panel { padding: 18px; }
  .actions button { flex: 1 1 140px; }
}