:root, [data-theme="dark"] {
  --bg-main: #0f172a;
  --bg-card: rgba(30, 41, 59, 0.85);
  --bg-card-solid: #1e293b;
  --bg-input: rgba(15, 23, 42, 0.6);
  --glass-border: rgba(255, 255, 255, 0.08);
  --text-primary: #f8fafc;
  --text-secondary: #cbd5e1;
  --text-muted: #94a3b8;
  --accent: #6366f1;
  --accent-hover: #818cf8;
  --success: #10b981;
  --error: #f43f5e;
  --warning: #f59e0b;
  --shadow: 0 10px 25px -5px rgba(0,0,0,0.3);
}

[data-theme="light"] {
  --bg-main: #f1f5f9;
  --bg-card: rgba(255, 255, 255, 0.9);
  --bg-card-solid: #ffffff;
  --bg-input: rgba(241, 245, 249, 0.8);
  --glass-border: rgba(0, 0, 0, 0.08);
  --text-primary: #0f172a;
  --text-secondary: #475569;
  --text-muted: #94a3b8;
  --accent: #4f46e5;
  --accent-hover: #6366f1;
  --success: #059669;
  --error: #e11d48;
  --warning: #d97706;
  --shadow: 0 10px 25px -5px rgba(0,0,0,0.08);
}

:root {
  --radius: 12px;
  --radius-sm: 8px;
  --transition: 0.2s cubic-bezier(0.4, 0, 0.2, 1);
}

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

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Noto Sans CJK SC", "Microsoft YaHei", sans-serif;
  background: var(--bg-main);
  color: var(--text-primary);
  min-height: 100vh;
  line-height: 1.6;
}

/* Layout */
.app { max-width: 1400px; margin: 0 auto; padding: 16px; }

/* Three-column layout for practice/browse */
.layout-3col {
  display: grid;
  grid-template-columns: 240px 1fr 400px;
  gap: 16px;
  align-items: start;
}
.layout-3col .sidebar { position: sticky; top: 16px; }
.layout-3col .main-area { min-width: 0; }
.layout-3col .page-ref { position: sticky; top: 16px; }
.layout-3col .page-ref img { width: 100%; border-radius: var(--radius-sm); border: 1px solid var(--glass-border); }

/* Sidebar toggle */
.sidebar { transition: var(--transition); }
.sidebar.collapsed { width: 0; overflow: hidden; opacity: 0; padding: 0; margin: 0; }
.sidebar-toggle {
  background: var(--bg-card); border: 1px solid var(--glass-border);
  color: var(--text-muted); padding: 4px 8px; border-radius: var(--radius-sm);
  cursor: pointer; font-size: 0.8rem; position: absolute; top: 8px; left: -30px;
  z-index: 10;
}

@media (max-width: 1200px) {
  .layout-3col { grid-template-columns: 200px 1fr 300px; }
}
@media (max-width: 960px) {
  .layout-3col { grid-template-columns: 1fr; }
  .layout-3col .sidebar { display: none; }
  .layout-3col .page-ref { display: none; }
}
@media (max-width: 768px) {
  .app { padding: 10px; }
  header { flex-direction: column; gap: 8px; }
}

header {
  display: flex; align-items: center; justify-content: space-between;
  padding: 12px 0; margin-bottom: 20px; border-bottom: 1px solid var(--glass-border);
}
header h1 { font-size: 1.3rem; background: linear-gradient(135deg, #6366f1, #a78bfa); -webkit-background-clip: text; -webkit-text-fill-color: transparent; }
header nav { display: flex; gap: 8px; }
header nav button {
  background: var(--bg-card); border: 1px solid var(--glass-border); color: var(--text-secondary);
  padding: 6px 14px; border-radius: var(--radius-sm); cursor: pointer; font-size: 0.85rem;
  transition: var(--transition);
}
header nav button:hover, header nav button.active { background: var(--accent); color: #fff; border-color: var(--accent); }

/* Cards */
.card {
  background: var(--bg-card); backdrop-filter: blur(12px);
  border: 1px solid var(--glass-border); border-radius: var(--radius);
  padding: 20px; margin-bottom: 16px; box-shadow: var(--shadow);
}
.card h2 { font-size: 1.1rem; margin-bottom: 12px; color: var(--text-primary); }
.card h3 { font-size: 0.95rem; margin-bottom: 8px; color: var(--text-secondary); }

/* Stats row */
.stats { display: flex; gap: 12px; flex-wrap: wrap; margin-bottom: 16px; }
.stat-card {
  flex: 1; min-width: 100px; text-align: center;
  background: var(--bg-card); border: 1px solid var(--glass-border);
  border-radius: var(--radius-sm); padding: 14px 10px;
}
.stat-card .num { font-size: 1.6rem; font-weight: 700; color: var(--accent); }
.stat-card .label { font-size: 0.75rem; color: var(--text-muted); margin-top: 2px; }

/* Buttons */
.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: 6px;
  padding: 10px 20px; border-radius: var(--radius-sm); border: none;
  font-size: 0.9rem; font-weight: 500; cursor: pointer; transition: var(--transition);
}
.btn-primary { background: var(--accent); color: #fff; }
.btn-primary:hover { background: var(--accent-hover); transform: translateY(-1px); }
.btn-secondary { background: var(--bg-card-solid); color: var(--text-secondary); border: 1px solid var(--glass-border); }
.btn-secondary:hover { border-color: var(--accent); color: var(--text-primary); }
.btn-success { background: var(--success); color: #fff; }
.btn-error { background: var(--error); color: #fff; }
.btn-sm { padding: 6px 12px; font-size: 0.8rem; }
.btn-block { width: 100%; }
.btn:disabled { opacity: 0.5; cursor: not-allowed; }

/* Chips / Tags */
.chips { display: flex; flex-wrap: wrap; gap: 8px; margin-bottom: 12px; }
.chip {
  padding: 6px 14px; border-radius: 20px; font-size: 0.8rem; cursor: pointer;
  background: var(--bg-input); border: 1px solid var(--glass-border); color: var(--text-secondary);
  transition: var(--transition);
}
.chip.selected { background: var(--accent); color: #fff; border-color: var(--accent); }
.chip:hover { border-color: var(--accent); }

/* Input */
.input-group { margin-bottom: 12px; }
.input-group label { display: block; font-size: 0.8rem; color: var(--text-muted); margin-bottom: 4px; }
input[type="text"], input[type="number"] {
  width: 100%; padding: 10px 14px; background: var(--bg-input);
  border: 1px solid var(--glass-border); border-radius: var(--radius-sm);
  color: var(--text-primary); font-size: 1rem; outline: none; transition: var(--transition);
}
input:focus { border-color: var(--accent); }

/* Problem display */
.problem-header { display: flex; justify-content: space-between; align-items: center; margin-bottom: 12px; font-size: 0.85rem; color: var(--text-muted); }
.problem-image { width: 100%; max-width: 520px; border-radius: var(--radius-sm); margin-bottom: 12px; border: 1px solid var(--glass-border); }
.problem-text { font-size: 1.05rem; line-height: 1.8; margin-bottom: 16px; white-space: pre-wrap; }
.problem-meta { display: flex; gap: 8px; margin-bottom: 12px; }
.tag { padding: 2px 8px; border-radius: 4px; font-size: 0.75rem; }
.tag-topic { background: rgba(99,102,241,0.2); color: #a5b4fc; }
.tag-diff { background: rgba(16,185,129,0.2); color: #6ee7b7; }
.tag-stage { background: rgba(245,158,11,0.2); color: #fcd34d; }

/* Answer area */
.answer-area { margin: 16px 0; }
.answer-input { font-size: 1.1rem; padding: 12px; }
.choice-btns { display: flex; gap: 10px; flex-wrap: wrap; }
.choice-btn {
  padding: 10px 24px; border-radius: var(--radius-sm); border: 2px solid var(--glass-border);
  background: var(--bg-input); color: var(--text-primary); font-size: 1rem; cursor: pointer;
  transition: var(--transition); min-width: 60px; text-align: center;
}
.choice-btn:hover { border-color: var(--accent); }
.choice-btn.selected { border-color: var(--accent); background: rgba(99,102,241,0.2); }

/* Result */
.result-bar {
  padding: 12px 16px; border-radius: var(--radius-sm); margin-bottom: 12px;
  font-weight: 500; display: flex; align-items: center; gap: 8px;
}
.result-correct { background: rgba(16,185,129,0.15); color: var(--success); border: 1px solid rgba(16,185,129,0.3); }
.result-wrong { background: rgba(244,63,94,0.15); color: var(--error); border: 1px solid rgba(244,63,94,0.3); }

.solution-box {
  background: var(--bg-input); border: 1px solid var(--glass-border);
  border-radius: var(--radius-sm); padding: 14px; margin: 12px 0; font-size: 0.9rem;
  line-height: 1.7; white-space: pre-wrap;
}

/* Progress bar */
.progress-bar { display: flex; gap: 4px; margin: 12px 0; }
.progress-dot { width: 10px; height: 10px; border-radius: 50%; background: var(--glass-border); }
.progress-dot.done { background: var(--success); }
.progress-dot.wrong { background: var(--error); }
.progress-dot.current { background: var(--accent); box-shadow: 0 0 6px var(--accent); }

/* Timer */
.timer { font-size: 1.1rem; font-weight: 600; color: var(--accent); font-variant-numeric: tabular-nums; }

/* Bar chart */
.bar-chart { margin: 12px 0; }
.bar-row { display: flex; align-items: center; gap: 8px; margin-bottom: 8px; }
.bar-label { width: 60px; font-size: 0.8rem; color: var(--text-muted); text-align: right; }
.bar-track { flex: 1; height: 20px; background: var(--bg-input); border-radius: 10px; overflow: hidden; }
.bar-fill { height: 100%; border-radius: 10px; transition: width 0.5s ease; }
.bar-value { width: 80px; font-size: 0.8rem; color: var(--text-secondary); }

/* Tree view (browse) */
.tree { font-size: 0.9rem; }
.tree-item { padding: 6px 0; cursor: pointer; display: flex; justify-content: space-between; align-items: center; }
.tree-item:hover { color: var(--accent); }
.tree-children { padding-left: 20px; }
.tree-toggle { display: inline-block; width: 16px; color: var(--text-muted); }

/* Checkbox */
.checkbox-row { display: flex; align-items: center; gap: 8px; margin-bottom: 8px; font-size: 0.85rem; color: var(--text-secondary); }
.checkbox-row input[type="checkbox"] { accent-color: var(--accent); width: 16px; height: 16px; }

/* Canvas scratchpad */
.scratchpad {
  position: fixed; bottom: 0; left: 0; right: 0; height: 200px;
  background: var(--bg-card-solid); border-top: 1px solid var(--glass-border);
  z-index: 100; display: none; flex-direction: column;
}
.scratchpad.open { display: flex; }
.scratchpad-bar { display: flex; justify-content: space-between; align-items: center; padding: 4px 12px; }
.scratchpad canvas { flex: 1; touch-action: none; cursor: crosshair; }

/* Share modal */
.modal-overlay {
  position: fixed; inset: 0; background: rgba(0,0,0,0.6); z-index: 200;
  display: none; align-items: center; justify-content: center;
}
.modal-overlay.open { display: flex; }
.modal { background: var(--bg-card-solid); border-radius: var(--radius); padding: 24px; max-width: 400px; width: 90%; }

/* Responsive */
@media (max-width: 768px) {
  .stats { gap: 8px; }
  .stat-card { min-width: 80px; padding: 10px 6px; }
  .stat-card .num { font-size: 1.3rem; }
  .problem-text { font-size: 0.95rem; }
}

/* Image viewer modal */
.img-viewer-overlay {
  position: fixed; inset: 0; background: rgba(0,0,0,0.85); z-index: 300;
  display: none; align-items: center; justify-content: center; cursor: grab;
  touch-action: none;
}
.img-viewer-overlay.open { display: flex; }
.img-viewer-overlay.dragging { cursor: grabbing; }
.img-viewer-overlay img {
  max-width: 90vw; max-height: 90vh; object-fit: contain;
  transition: transform 0.1s ease; user-select: none; -webkit-user-drag: none;
}
.img-viewer-controls {
  position: fixed; bottom: 20px; left: 50%; transform: translateX(-50%);
  display: flex; gap: 8px; z-index: 301;
  background: var(--bg-card-solid); padding: 8px 12px; border-radius: 24px;
  border: 1px solid var(--glass-border);
}
.img-viewer-controls button {
  background: var(--bg-input); border: 1px solid var(--glass-border);
  color: var(--text-primary); width: 36px; height: 36px; border-radius: 50%;
  cursor: pointer; font-size: 1rem; display: flex; align-items: center; justify-content: center;
}
.img-viewer-controls button:hover { border-color: var(--accent); }

/* Page ref clickable */
.page-ref img { cursor: zoom-in; transition: var(--transition); }
.page-ref img:hover { border-color: var(--accent); box-shadow: 0 0 12px rgba(99,102,241,0.3); }

/* Hidden */
.hidden { display: none !important; }