:root {
  --bg: #0d1117;
  --bg2: #161b22;
  --bg3: #21262d;
  --border: #30363d;
  --text: #e6edf3;
  --text2: #8b949e;
  --accent: #58a6ff;
  --green: #3fb950;
  --red: #f85149;
  --yellow: #d29922;
  --sidebar-w: 240px;
  --topbar-h: 56px;
  --radius: 8px;
}

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

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  background: var(--bg);
  color: var(--text);
  height: 100vh;
  display: flex;
  overflow: hidden;
}

/* ── Sidebar ────────────────────────────────────── */
.sidebar {
  width: var(--sidebar-w);
  background: var(--bg2);
  border-right: 1px solid var(--border);
  display: flex;
  flex-direction: column;
  flex-shrink: 0;
  height: 100vh;
  overflow-y: auto;
  transition: transform 0.25s ease;
  z-index: 100;
}

.sidebar-header {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 16px 14px;
  border-bottom: 1px solid var(--border);
  flex-shrink: 0;
}

.logo {
  background: var(--accent);
  color: #000;
  font-weight: 700;
  font-size: 13px;
  padding: 3px 7px;
  border-radius: 4px;
}

.logo-text {
  font-size: 13px;
  font-weight: 600;
  color: var(--text);
  flex: 1;
}

.sidebar-header .hamburger {
  display: none;
  background: none;
  border: none;
  color: var(--text2);
  cursor: pointer;
  font-size: 16px;
  padding: 2px;
}

.chapter-list { flex: 1; padding: 8px 0; }

.chapter-item {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 9px 14px;
  cursor: pointer;
  border-left: 2px solid transparent;
  transition: background 0.15s;
  font-size: 13px;
  color: var(--text2);
}

.chapter-item:hover { background: var(--bg3); color: var(--text); }

.chapter-item.active {
  background: rgba(88,166,255,0.1);
  border-left-color: var(--accent);
  color: var(--accent);
}

.chapter-icon { font-size: 16px; width: 22px; text-align: center; }

.chapter-info { flex: 1; min-width: 0; }

.chapter-name { white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }

.chapter-meta { font-size: 11px; margin-top: 2px; color: var(--text2); }

.chapter-item.active .chapter-meta { color: rgba(88,166,255,0.7); }

.chapter-progress-bar {
  height: 2px;
  background: var(--bg3);
  border-radius: 1px;
  margin-top: 4px;
  overflow: hidden;
}

.chapter-progress-fill {
  height: 100%;
  background: var(--green);
  border-radius: 1px;
  transition: width 0.3s;
}

.sidebar-footer {
  padding: 12px 14px;
  border-top: 1px solid var(--border);
  flex-shrink: 0;
}

.overall-progress .progress-label {
  display: flex;
  justify-content: space-between;
  font-size: 11px;
  color: var(--text2);
  margin-bottom: 6px;
}

/* ── Main ────────────────────────────────────────── */
.main {
  flex: 1;
  display: flex;
  flex-direction: column;
  min-width: 0;
  height: 100vh;
  overflow: hidden;
}

.topbar {
  height: var(--topbar-h);
  background: var(--bg2);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  padding: 0 16px;
  gap: 12px;
  flex-shrink: 0;
}

#open-sidebar.hamburger {
  display: none;
  background: none;
  border: none;
  color: var(--text2);
  cursor: pointer;
  font-size: 20px;
  padding: 4px;
  line-height: 1;
}

.topbar h1 {
  font-size: 15px;
  font-weight: 600;
  flex: 1;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.mode-tabs { display: flex; gap: 4px; flex-shrink: 0; }

.tab-btn {
  background: none;
  border: 1px solid var(--border);
  color: var(--text2);
  padding: 5px 12px;
  border-radius: 6px;
  cursor: pointer;
  font-size: 13px;
  transition: all 0.15s;
}

.tab-btn:hover { background: var(--bg3); color: var(--text); }

.tab-btn.active {
  background: rgba(88,166,255,0.15);
  border-color: var(--accent);
  color: var(--accent);
}

.add-btn {
  background: none;
  border: 1px solid var(--green);
  color: var(--green);
  padding: 5px 12px;
  border-radius: 6px;
  cursor: pointer;
  font-size: 13px;
  white-space: nowrap;
  transition: background 0.15s;
}

.add-btn:hover { background: rgba(63,185,80,0.12); }

/* ── Screens ─────────────────────────────────────── */
.screen { display: none; flex-direction: column; height: calc(100vh - var(--topbar-h)); overflow-y: auto; }
.screen.active { display: flex; }

/* Welcome */
.welcome {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  flex: 1;
  gap: 12px;
  text-align: center;
  padding: 40px 20px;
}

.welcome-icon { font-size: 64px; }
.welcome h2 { font-size: 22px; }
.welcome p { color: var(--text2); font-size: 15px; }
.welcome-stats { color: var(--accent) !important; font-weight: 500; }

/* Card Progress Bar */
.card-progress {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 12px 20px;
  flex-shrink: 0;
  font-size: 13px;
  color: var(--text2);
}

.card-progress .progress-bar { flex: 1; }

/* Progress Bar shared */
.progress-bar {
  height: 4px;
  background: var(--bg3);
  border-radius: 2px;
  overflow: hidden;
}

.progress-bar > div {
  height: 100%;
  background: var(--accent);
  border-radius: 2px;
  transition: width 0.4s ease;
}

/* Flashcard */
.flashcard-area {
  flex: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 20px;
  perspective: 1200px;
}

.flashcard {
  width: 100%;
  max-width: 640px;
  min-height: 280px;
  cursor: pointer;
  outline: none;
}

.flashcard:focus-visible .flashcard-inner { box-shadow: 0 0 0 2px var(--accent); }

.flashcard-inner {
  position: relative;
  width: 100%;
  min-height: 280px;
  transition: transform 0.5s ease;
  transform-style: preserve-3d;
  border-radius: 14px;
}

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

.flashcard-front,
.flashcard-back {
  position: absolute;
  inset: 0;
  backface-visibility: hidden;
  border-radius: 14px;
  padding: 32px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  text-align: center;
  gap: 12px;
}

.flashcard-front {
  background: var(--bg2);
  border: 1px solid var(--border);
}

.flashcard-back {
  background: #0f1e10;
  border: 1px solid var(--green);
  transform: rotateY(180deg);
}

.card-label {
  font-size: 10px;
  text-transform: uppercase;
  letter-spacing: 2px;
  color: var(--accent);
}

.answer-label { color: var(--green); }

.card-text {
  font-size: 16px;
  line-height: 1.6;
  color: var(--text);
}

.card-hint {
  font-size: 12px;
  color: var(--border);
  margin-top: 8px;
}

/* Card actions */
.card-actions {
  display: flex;
  gap: 16px;
  justify-content: center;
  padding: 16px 20px 24px;
  flex-shrink: 0;
}

.btn-wrong,
.btn-correct {
  padding: 12px 36px;
  border-radius: 8px;
  font-size: 15px;
  font-weight: 500;
  cursor: pointer;
  border: 1px solid;
  transition: background 0.15s;
}

.btn-wrong {
  background: #2d1515;
  border-color: var(--red);
  color: var(--red);
}

.btn-wrong:hover { background: rgba(248,81,73,0.2); }

.btn-correct {
  background: #0f2318;
  border-color: var(--green);
  color: var(--green);
}

.btn-correct:hover { background: rgba(63,185,80,0.2); }

/* Done screen */
.chapter-done {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
  padding: 40px 20px;
  text-align: center;
}

.done-icon { font-size: 60px; }

/* Quiz */
.quiz-area {
  display: flex;
  flex-direction: column;
  gap: 16px;
  padding: 20px;
  max-width: 640px;
  width: 100%;
  margin: 0 auto;
}

.quiz-question {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 20px 24px;
  font-size: 16px;
  line-height: 1.5;
  min-height: 80px;
}

.quiz-options { display: flex; flex-direction: column; gap: 8px; }

.quiz-option {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 12px 16px;
  cursor: pointer;
  text-align: left;
  font-size: 14px;
  color: var(--text);
  line-height: 1.4;
  transition: background 0.15s, border-color 0.15s;
}

.quiz-option:hover:not(:disabled) { background: var(--bg2); border-color: var(--accent); }
.quiz-option.correct { background: #0f2318; border-color: var(--green); color: var(--green); }
.quiz-option.wrong { background: #2d1515; border-color: var(--red); color: var(--red); }
.quiz-option:disabled { cursor: default; }

.quiz-feedback {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 12px 16px;
  font-size: 13px;
  color: var(--text2);
}

.quiz-result {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
  padding: 40px 20px;
  text-align: center;
  margin: 0 auto;
  max-width: 400px;
}

/* Buttons shared */
.btn-primary {
  background: var(--accent);
  color: #000;
  border: none;
  padding: 10px 24px;
  border-radius: 6px;
  font-size: 14px;
  font-weight: 600;
  cursor: pointer;
  transition: opacity 0.15s;
}

.btn-primary:hover { opacity: 0.85; }

.btn-secondary {
  background: none;
  border: 1px solid var(--border);
  color: var(--text2);
  padding: 5px 12px;
  border-radius: 6px;
  font-size: 12px;
  cursor: pointer;
  transition: background 0.15s;
}

.btn-secondary:hover { background: var(--bg3); color: var(--text); }

.btn-sm { font-size: 11px; padding: 4px 10px; }

/* Modal */
.modal-overlay {
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.7);
  display: flex;
  align-items: center;
  justify-content: center;
  z-index: 200;
  padding: 20px;
}

.modal {
  background: var(--bg2);
  border: 1px solid var(--border);
  border-radius: 12px;
  width: 100%;
  max-width: 480px;
}

.modal-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 16px 20px;
  border-bottom: 1px solid var(--border);
}

.modal-header h3 { font-size: 15px; }

.modal-close {
  background: none;
  border: none;
  color: var(--text2);
  cursor: pointer;
  font-size: 18px;
}

.modal-body { padding: 16px 20px; display: flex; flex-direction: column; gap: 12px; }

.modal-body label {
  display: flex;
  flex-direction: column;
  gap: 5px;
  font-size: 13px;
  color: var(--text2);
}

.modal-body input,
.modal-body select,
.modal-body textarea {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  color: var(--text);
  padding: 8px 10px;
  font-size: 14px;
  font-family: inherit;
  resize: vertical;
}

.modal-body input:focus,
.modal-body select:focus,
.modal-body textarea:focus {
  outline: none;
  border-color: var(--accent);
}

.modal-footer {
  display: flex;
  justify-content: flex-end;
  gap: 8px;
  padding: 12px 20px;
  border-top: 1px solid var(--border);
}

/* Toast */
.toast {
  position: fixed;
  bottom: 24px;
  left: 50%;
  transform: translateX(-50%);
  background: var(--bg2);
  border: 1px solid var(--border);
  color: var(--text);
  padding: 10px 20px;
  border-radius: 8px;
  font-size: 14px;
  z-index: 300;
  box-shadow: 0 4px 20px rgba(0,0,0,0.4);
  animation: fadeInUp 0.2s ease;
}

@keyframes fadeInUp {
  from { opacity: 0; transform: translateX(-50%) translateY(8px); }
  to   { opacity: 1; transform: translateX(-50%) translateY(0); }
}

/* Overlay for mobile sidebar */
.overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.5);
  z-index: 50;
}

/* ── Responsive ──────────────────────────────────── */
@media (max-width: 700px) {
  .sidebar {
    position: fixed;
    top: 0;
    left: 0;
    height: 100%;
    transform: translateX(-100%);
  }

  .sidebar.open { transform: translateX(0); }
  .sidebar-header .hamburger { display: block; }
  .overlay.visible { display: block; }
  #open-sidebar.hamburger { display: block; }

  .topbar h1 { font-size: 13px; }
  .mode-tabs .tab-btn { padding: 5px 8px; font-size: 12px; }
  .add-btn { padding: 5px 8px; font-size: 12px; }

  .flashcard-area { padding: 12px; }
  .flashcard-front, .flashcard-back { padding: 24px 20px; }
  .card-text { font-size: 14px; }
}