:root {
  --bg: #f4f6fb;
  --bg-soft: #eceff6;
  --bg-elevated: #ffffff;
  --text: #171b26;
  --text-muted: #656e82;
  --border: #e4e8f1;
  --border-soft: #edf0f7;

  --accent: #4f6bff;
  --accent-2: #7c5cff;
  --accent-hover: #3d59f5;
  --accent-soft: #eef1ff;
  --accent-grad: linear-gradient(135deg, #4f6bff 0%, #7c5cff 100%);

  --green: #17a865;
  --green-soft: #e7f8ef;
  --red: #e5484d;
  --red-soft: #fdeced;
  --amber: #b8790a;
  --amber-soft: #fdf1d9;

  --radius-lg: 18px;
  --radius-md: 12px;
  --radius-sm: 9px;
  --shadow-sm: 0 1px 2px rgba(23, 27, 38, 0.05), 0 1px 1px rgba(23, 27, 38, 0.04);
  --shadow-md: 0 4px 16px rgba(23, 27, 38, 0.07), 0 1px 3px rgba(23, 27, 38, 0.05);
  --shadow-lg: 0 16px 40px rgba(23, 27, 38, 0.14), 0 3px 10px rgba(23, 27, 38, 0.06);
  color-scheme: light dark;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0d0f16;
    --bg-soft: #12151f;
    --bg-elevated: #171b27;
    --text: #edeff5;
    --text-muted: #9199ac;
    --border: #262c3d;
    --border-soft: #1f2432;

    --accent: #7c93ff;
    --accent-2: #a78bff;
    --accent-hover: #97a9ff;
    --accent-soft: #1c2140;
    --accent-grad: linear-gradient(135deg, #5f7dff 0%, #9b7bff 100%);

    --green: #3ddb8a;
    --green-soft: #12281f;
    --red: #ff6b70;
    --red-soft: #341a1c;
    --amber: #e8b84b;
    --amber-soft: #332a12;

    --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.3);
    --shadow-md: 0 4px 18px rgba(0, 0, 0, 0.4);
    --shadow-lg: 0 20px 50px rgba(0, 0, 0, 0.55);
  }
}

:root[data-theme="dark"] {
  --bg: #0d0f16;
  --bg-soft: #12151f;
  --bg-elevated: #171b27;
  --text: #edeff5;
  --text-muted: #9199ac;
  --border: #262c3d;
  --border-soft: #1f2432;

  --accent: #7c93ff;
  --accent-2: #a78bff;
  --accent-hover: #97a9ff;
  --accent-soft: #1c2140;
  --accent-grad: linear-gradient(135deg, #5f7dff 0%, #9b7bff 100%);

  --green: #3ddb8a;
  --green-soft: #12281f;
  --red: #ff6b70;
  --red-soft: #341a1c;
  --amber: #e8b84b;
  --amber-soft: #332a12;

  --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.3);
  --shadow-md: 0 4px 18px rgba(0, 0, 0, 0.4);
  --shadow-lg: 0 20px 50px rgba(0, 0, 0, 0.55);
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}

a { color: var(--accent); }

.app-shell {
  display: flex;
  min-height: 100vh;
  background:
    radial-gradient(1100px 500px at 100% -10%, color-mix(in srgb, var(--accent) 10%, transparent), transparent 60%),
    var(--bg);
}

/* Sidebar */
.sidebar {
  width: 272px;
  flex-shrink: 0;
  background: var(--bg-elevated);
  border-right: 1px solid var(--border);
  display: flex;
  flex-direction: column;
  position: sticky;
  top: 0;
  height: 100vh;
  overflow-y: auto;
}

.sidebar-header {
  padding: 22px 20px 14px;
}

.sidebar-header .logo {
  font-size: 1.12rem;
  font-weight: 800;
  display: flex;
  align-items: center;
  gap: 10px;
  letter-spacing: -0.01em;
}

.sidebar-header .logo .logo-badge {
  width: 32px;
  height: 32px;
  border-radius: 10px;
  background: var(--accent-grad);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1rem;
  box-shadow: var(--shadow-sm);
}

.sidebar-header .subtitle {
  color: var(--text-muted);
  font-size: 0.8rem;
  margin-top: 6px;
}

.nav-section {
  padding: 6px 12px;
}

.nav-section-title {
  font-size: 0.7rem;
  text-transform: uppercase;
  letter-spacing: 0.07em;
  color: var(--text-muted);
  padding: 14px 10px 6px;
  font-weight: 700;
}

.nav-item {
  display: flex;
  align-items: center;
  gap: 10px;
  width: 100%;
  text-align: left;
  padding: 9px 10px;
  border-radius: var(--radius-sm);
  border: none;
  background: transparent;
  color: var(--text);
  font-size: 0.89rem;
  cursor: pointer;
  font-family: inherit;
  position: relative;
  transition: background 0.15s;
}

.nav-item:hover { background: var(--bg-soft); }
.nav-item.active { background: var(--accent-soft); color: var(--accent); font-weight: 700; }
.nav-item.active::before {
  content: "";
  position: absolute;
  left: -12px;
  top: 8px;
  bottom: 8px;
  width: 3px;
  border-radius: 3px;
  background: var(--accent-grad);
}

.nav-item .nav-icon { font-size: 1rem; width: 20px; text-align: center; }
.nav-item .nav-progress {
  margin-left: auto;
  font-size: 0.7rem;
  color: var(--text-muted);
  font-variant-numeric: tabular-nums;
}

.sidebar-footer {
  margin-top: auto;
  padding: 14px 18px 18px;
  border-top: 1px solid var(--border);
  display: flex;
  flex-direction: column;
  gap: 10px;
}

.theme-toggle {
  display: flex;
  gap: 4px;
  background: var(--bg-soft);
  border: 1px solid var(--border);
  border-radius: 999px;
  padding: 3px;
}
.theme-toggle button {
  flex: 1;
  border: none;
  background: transparent;
  padding: 6px 0;
  border-radius: 999px;
  cursor: pointer;
  font-size: 0.85rem;
  color: var(--text-muted);
  font-family: inherit;
}
.theme-toggle button.active {
  background: var(--bg-elevated);
  color: var(--text);
  box-shadow: var(--shadow-sm);
}

.footer-note {
  font-size: 0.72rem;
  color: var(--text-muted);
}

/* Main */
.main {
  flex: 1;
  min-width: 0;
  padding: 30px 40px 70px;
  max-width: 1020px;
}

.main.fade-in { animation: fadeIn 0.28s ease; }
@keyframes fadeIn {
  from { opacity: 0; transform: translateY(6px); }
  to { opacity: 1; transform: translateY(0); }
}

.topbar {
  position: sticky;
  top: 0;
  z-index: 10;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  margin: -14px -8px 24px;
  padding: 14px 8px;
  background: color-mix(in srgb, var(--bg) 78%, transparent);
  backdrop-filter: blur(10px);
  -webkit-backdrop-filter: blur(10px);
  flex-wrap: wrap;
}

.topbar h1 {
  font-size: 1.5rem;
  margin: 0;
  letter-spacing: -0.015em;
}

.pill-tabs {
  display: flex;
  gap: 4px;
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  padding: 4px;
  border-radius: 999px;
  box-shadow: var(--shadow-sm);
}

.pill-tab {
  padding: 8px 16px;
  border-radius: 999px;
  border: none;
  background: transparent;
  color: var(--text-muted);
  font-size: 0.85rem;
  cursor: pointer;
  font-family: inherit;
  font-weight: 600;
  transition: background 0.15s, color 0.15s;
}

.pill-tab.active {
  background: var(--accent-grad);
  color: #fff;
}

/* Hero */
.hero {
  border-radius: var(--radius-lg);
  padding: 28px 30px;
  margin-bottom: 24px;
  background: var(--accent-grad);
  color: #fff;
  box-shadow: var(--shadow-lg);
  position: relative;
  overflow: hidden;
}
.hero::after {
  content: "";
  position: absolute;
  inset: 0;
  background: radial-gradient(420px 220px at 90% -20%, rgba(255,255,255,0.25), transparent 60%);
}
.hero-inner { position: relative; }
.hero h1 { margin: 0 0 6px; font-size: 1.6rem; letter-spacing: -0.02em; }
.hero p { margin: 0; opacity: 0.92; font-size: 0.94rem; max-width: 560px; }

/* Dashboard */
.stat-row {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
  gap: 14px;
  margin-bottom: 26px;
}

.stat-card {
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  padding: 16px 18px;
  box-shadow: var(--shadow-sm);
}

.stat-card .value {
  font-size: 1.65rem;
  font-weight: 800;
  letter-spacing: -0.02em;
  background: var(--accent-grad);
  -webkit-background-clip: text;
  background-clip: text;
  color: transparent;
}

.stat-card .label {
  font-size: 0.78rem;
  color: var(--text-muted);
  margin-top: 3px;
}

.cta-row {
  display: flex;
  gap: 12px;
  margin-bottom: 28px;
  flex-wrap: wrap;
}

.btn {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 10px 18px;
  border-radius: 999px;
  border: 1px solid var(--border);
  background: var(--bg-elevated);
  color: var(--text);
  font-size: 0.89rem;
  font-weight: 600;
  cursor: pointer;
  font-family: inherit;
  transition: background 0.15s, transform 0.05s, box-shadow 0.15s;
  box-shadow: var(--shadow-sm);
}

.btn:hover { transform: translateY(-1px); box-shadow: var(--shadow-md); }
.btn:active { transform: translateY(0) scale(0.98); }

.btn-primary {
  background: var(--accent-grad);
  border-color: transparent;
  color: #fff;
}

.btn-green { background: var(--green); border-color: var(--green); color: #fff; }
.btn-red { background: var(--red-soft); border-color: var(--red); color: var(--red); }
.btn-sm { padding: 7px 14px; font-size: 0.82rem; }
.btn:disabled { opacity: 0.5; cursor: not-allowed; transform: none; box-shadow: none; }

.topic-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(290px, 1fr));
  gap: 16px;
}

.topic-card {
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 20px;
  cursor: pointer;
  box-shadow: var(--shadow-sm);
  transition: transform 0.15s, box-shadow 0.15s, border-color 0.15s;
  text-align: left;
  display: flex;
  flex-direction: column;
  gap: 10px;
}

.topic-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--shadow-lg);
  border-color: color-mix(in srgb, var(--accent) 35%, var(--border));
}

.topic-card-top {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 10px;
}

.topic-icon-badge {
  width: 44px;
  height: 44px;
  border-radius: 13px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.35rem;
  flex-shrink: 0;
}

.topic-card .title { font-weight: 700; margin: 2px 0 0; font-size: 1.02rem; letter-spacing: -0.01em; }
.topic-card .category {
  font-size: 0.68rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  padding: 3px 9px;
  border-radius: 999px;
  display: inline-block;
  width: fit-content;
}

.progress-caption {
  font-size: 0.75rem;
  color: var(--text-muted);
  display: flex;
  justify-content: space-between;
  margin-top: auto;
  padding-top: 4px;
}

/* Progress ring */
.ring { flex-shrink: 0; }
.ring-track { stroke: var(--border); }
.ring-fill { stroke: var(--accent); stroke-linecap: round; transition: stroke-dashoffset 0.4s ease; }
.ring-label {
  font-size: 0.62rem;
  font-weight: 800;
  fill: var(--text);
}

.progress-bar-track {
  height: 6px;
  background: var(--border);
  border-radius: 4px;
  overflow: hidden;
}
.progress-bar-fill {
  height: 100%;
  background: var(--accent-grad);
  border-radius: 4px;
  transition: width 0.3s;
}

/* Topic summary view */
.summary-content {
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 28px 32px;
  box-shadow: var(--shadow-sm);
}

.summary-content h4 {
  margin: 24px 0 8px;
  font-size: 1.02rem;
  color: var(--accent);
  letter-spacing: -0.01em;
}
.summary-content h4:first-child { margin-top: 0; }
.summary-content ul, .summary-content ol { padding-left: 22px; margin: 6px 0; }
.summary-content li { margin: 5px 0; }
.summary-content p { margin: 8px 0; }
.summary-content code {
  background: var(--accent-soft);
  padding: 1px 6px;
  border-radius: 5px;
  font-size: 0.88em;
  color: var(--accent);
}
.summary-content .hint {
  background: var(--amber-soft);
  border: 1px solid color-mix(in srgb, var(--amber) 50%, transparent);
  color: var(--text);
  padding: 12px 16px;
  border-radius: var(--radius-sm);
  font-size: 0.88rem;
  margin-top: 20px;
}
.port-table {
  border-collapse: collapse;
  margin: 8px 0;
  font-size: 0.88rem;
  border-radius: var(--radius-sm);
  overflow: hidden;
}
.port-table th, .port-table td {
  border: 1px solid var(--border);
  padding: 6px 14px;
  text-align: left;
}
.port-table th { background: var(--accent-soft); color: var(--accent); font-weight: 700; }

.notes-box {
  margin-top: 26px;
  border-top: 1px solid var(--border);
  padding-top: 18px;
}
.notes-box label {
  font-size: 0.85rem;
  font-weight: 700;
  color: var(--text-muted);
  display: block;
  margin-bottom: 8px;
}
.notes-box textarea {
  width: 100%;
  min-height: 110px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
  background: var(--bg);
  color: var(--text);
  padding: 11px 13px;
  font-family: inherit;
  font-size: 0.9rem;
  resize: vertical;
}
.notes-box textarea:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-soft);
}
.notes-saved {
  font-size: 0.72rem;
  color: var(--green);
  margin-top: 5px;
  height: 14px;
  font-weight: 600;
}

/* Flashcards */
.flash-toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: 18px;
  flex-wrap: wrap;
  gap: 10px;
}
.flash-toolbar .left { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
.flash-counter { font-size: 0.85rem; color: var(--text-muted); font-variant-numeric: tabular-nums; }

.flashcard-wrap {
  perspective: 1400px;
  max-width: 580px;
  margin: 0 auto;
  height: 320px;
}
.flashcard {
  position: relative;
  width: 100%;
  height: 100%;
  cursor: pointer;
  transition: transform 0.5s cubic-bezier(.4,.2,.2,1);
  transform-style: preserve-3d;
}
.flashcard.flipped { transform: rotateY(180deg); }
.flashcard-face {
  position: absolute;
  inset: 0;
  backface-visibility: hidden;
  border-radius: var(--radius-lg);
  border: 1px solid var(--border);
  background: var(--bg-elevated);
  box-shadow: var(--shadow-lg);
  display: flex;
  align-items: center;
  justify-content: center;
  text-align: center;
  padding: 34px;
  font-size: 1.15rem;
}
.flashcard-face.back {
  transform: rotateY(180deg);
  background: var(--accent-grad);
  color: #fff;
  font-size: 1.02rem;
}
.flashcard-face .face-label {
  position: absolute;
  top: 16px;
  left: 20px;
  font-size: 0.68rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
  font-weight: 700;
}
.flashcard-face.back .face-label { color: rgba(255,255,255,0.75); }

.flash-controls {
  display: flex;
  justify-content: center;
  gap: 14px;
  margin-top: 24px;
}

.flash-hint {
  text-align: center;
  color: var(--text-muted);
  font-size: 0.8rem;
  margin-top: 12px;
}

/* Quiz */
.quiz-progress {
  font-size: 0.85rem;
  color: var(--text-muted);
  margin-bottom: 10px;
  font-weight: 600;
}
.quiz-card {
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 28px 30px;
  box-shadow: var(--shadow-sm);
}
.quiz-source {
  font-size: 0.7rem;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: 8px;
  font-weight: 800;
}
.quiz-question {
  font-size: 1.1rem;
  font-weight: 700;
  margin-bottom: 20px;
  letter-spacing: -0.01em;
}
.quiz-options { display: flex; flex-direction: column; gap: 10px; }
.quiz-option {
  text-align: left;
  padding: 13px 16px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
  background: var(--bg);
  color: var(--text);
  font-family: inherit;
  font-size: 0.92rem;
  cursor: pointer;
  transition: border-color 0.15s, background 0.15s, transform 0.1s;
}
.quiz-option:hover:not(:disabled) { border-color: var(--accent); transform: translateX(2px); }
.quiz-option.correct { background: var(--green-soft); border-color: var(--green); font-weight: 700; }
.quiz-option.incorrect { background: var(--red-soft); border-color: var(--red); font-weight: 700; }
.quiz-option:disabled { cursor: default; }

.quiz-explain {
  margin-top: 18px;
  padding: 14px 16px;
  border-radius: var(--radius-sm);
  background: var(--accent-soft);
  font-size: 0.88rem;
  animation: fadeIn 0.2s ease;
}

.quiz-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-top: 22px;
}

.result-screen {
  text-align: center;
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 44px 30px;
  box-shadow: var(--shadow-sm);
}
.result-screen .big-score {
  font-size: 3.2rem;
  font-weight: 800;
  letter-spacing: -0.03em;
  background: var(--accent-grad);
  -webkit-background-clip: text;
  background-clip: text;
  color: transparent;
}
.result-screen .score-label { color: var(--text-muted); margin-bottom: 20px; }
.result-breakdown {
  text-align: left;
  max-width: 440px;
  margin: 24px auto 0;
}
.result-breakdown .row {
  display: flex;
  justify-content: space-between;
  padding: 7px 0;
  border-bottom: 1px solid var(--border);
  font-size: 0.86rem;
}

/* eBook / print view */
.ebook {
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 32px 36px;
  box-shadow: var(--shadow-sm);
}
.ebook .ebook-topic { margin-bottom: 36px; padding-bottom: 28px; border-bottom: 1px dashed var(--border); }
.ebook .ebook-topic:last-child { border-bottom: none; }
.ebook h2.ebook-h { font-size: 1.3rem; margin-bottom: 2px; letter-spacing: -0.01em; }
.ebook .ebook-cat { color: var(--text-muted); font-size: 0.8rem; margin-bottom: 16px; }

@media print {
  .sidebar, .topbar, .cta-row, .no-print { display: none !important; }
  .main { max-width: 100%; padding: 0; }
  body { background: #fff; color: #000; }
  .ebook { border: none; box-shadow: none; padding: 0; }
}

/* Mobile */
.menu-toggle { display: none; }

@media (max-width: 860px) {
  .sidebar {
    position: fixed;
    left: 0; top: 0; bottom: 0;
    transform: translateX(-100%);
    transition: transform 0.2s;
    z-index: 50;
    box-shadow: var(--shadow-lg);
  }
  .sidebar.open { transform: translateX(0); }
  .menu-toggle {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 38px; height: 38px;
    border-radius: var(--radius-sm);
    border: 1px solid var(--border);
    background: var(--bg-elevated);
    cursor: pointer;
    font-size: 1.1rem;
  }
  .main { padding: 20px 16px 50px; }
  .topbar { margin: -14px -4px 20px; padding: 14px 4px; }
  .sidebar-scrim {
    display: none;
    position: fixed; inset: 0; background: rgba(10,12,20,0.45); z-index: 40;
  }
  .sidebar-scrim.show { display: block; }
}

.badge {
  display: inline-block;
  padding: 2px 8px;
  border-radius: 999px;
  font-size: 0.7rem;
  font-weight: 700;
}
.badge-green { background: var(--green-soft); color: var(--green); }
.badge-amber { background: var(--amber-soft); color: var(--amber); }
.badge-muted { background: var(--border); color: var(--text-muted); }

.search-box {
  width: 100%;
  max-width: 340px;
  padding: 10px 14px;
  border-radius: 999px;
  border: 1px solid var(--border);
  background: var(--bg-elevated);
  color: var(--text);
  font-family: inherit;
  font-size: 0.88rem;
  box-shadow: var(--shadow-sm);
}
.search-box:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-soft);
}

.empty-state {
  text-align: center;
  padding: 60px 20px;
  color: var(--text-muted);
}
.empty-state .big { font-size: 2.4rem; margin-bottom: 10px; }