:root {
  color-scheme: light dark;
  --font-sans: 'Inter', 'Segoe UI', system-ui, -apple-system, BlinkMacSystemFont, 'Roboto', 'Ubuntu', sans-serif;
  --radius-lg: 18px;
  --radius-md: 12px;
  --radius-sm: 10px;
  --shadow-strong: 0 24px 55px rgba(15, 23, 42, 0.3);
  --shadow-soft: 0 12px 30px rgba(15, 23, 42, 0.18);
  --transition-base: 180ms ease;
}

body,
body[data-theme='dark'] {
  --color-page: #040b1a;
  --color-backdrop: radial-gradient(950px 600px at 12% 18%, rgba(99, 102, 241, 0.32), transparent),
    radial-gradient(900px 600px at 88% 12%, rgba(56, 189, 248, 0.28), transparent),
    linear-gradient(160deg, #050b18 0%, #0b1326 40%, #121d36 100%);
  --color-topbar: rgba(8, 14, 26, 0.82);
  --color-surface: rgba(15, 22, 40, 0.92);
  --color-surface-alt: rgba(19, 28, 48, 0.78);
  --color-stroke: rgba(99, 102, 241, 0.22);
  --color-stroke-strong: rgba(129, 140, 248, 0.6);
  --color-text-primary: #f8fafc;
  --color-text-secondary: #cbd5f5;
  --color-muted: #9aa5c2;
  --color-border: rgba(148, 163, 184, 0.22);
  --color-border-soft: rgba(148, 163, 184, 0.16);
  --color-primary: #6366f1;
  --color-primary-strong: #4f46e5;
  --color-primary-soft: rgba(99, 102, 241, 0.12);
  --color-accent: #34d399;
  --color-danger: #f87171;
  --color-warning: #fbbf24;
  --shadow-elevated: 0 28px 70px rgba(2, 6, 23, 0.55);
  --brand-icon-fill: #f8fafc;
}

body[data-theme='light'] {
  --color-page: #f6f8ff;
  --color-backdrop: radial-gradient(900px 560px at 10% 18%, rgba(79, 70, 229, 0.18), transparent),
    radial-gradient(780px 540px at 85% 10%, rgba(14, 165, 233, 0.18), transparent),
    linear-gradient(160deg, #ffffff 0%, #eef2ff 55%, #e2f3ff 100%);
  --color-topbar: rgba(255, 255, 255, 0.88);
  --color-surface: rgba(255, 255, 255, 0.92);
  --color-surface-alt: rgba(255, 255, 255, 0.78);
  --color-stroke: rgba(148, 163, 184, 0.18);
  --color-stroke-strong: rgba(99, 102, 241, 0.35);
  --color-text-primary: #1e293b;
  --color-text-secondary: #334155;
  --color-muted: #64748b;
  --color-border: rgba(148, 163, 184, 0.28);
  --color-border-soft: rgba(148, 163, 184, 0.18);
  --color-primary: #4f46e5;
  --color-primary-strong: #4338ca;
  --color-primary-soft: rgba(79, 70, 229, 0.12);
  --color-accent: #10b981;
  --color-danger: #dc2626;
  --color-warning: #f59e0b;
  --shadow-elevated: 0 26px 60px rgba(15, 23, 42, 0.18);
  --brand-icon-fill: #312e81;
}

* { box-sizing: border-box; }

html, body { height: 100%; }

body {
  margin: 0;
  font: 16px/1.6 var(--font-sans);
  background: var(--color-page);
  background-attachment: fixed;
  color: var(--color-text-primary);
  transition: background 320ms ease, color var(--transition-base);
}

body::before {
  content: '';
  position: fixed;
  inset: 0;
  pointer-events: none;
  background: var(--color-backdrop);
  z-index: -1;
}

.hidden { display: none !important; }

main { display: block; }

.topbar {
  position: sticky;
  top: 0;
  z-index: 40;
  display: grid;
  gap: 20px;
  padding: 18px clamp(16px, 5vw, 42px);
  background: var(--color-topbar);
  backdrop-filter: blur(14px) saturate(135%);
  border-bottom: 1px solid var(--color-border);
}

.brand {
  display: grid;
  grid-template-columns: auto minmax(0, 1fr);
  gap: 16px;
  align-items: center;
}

.brand-mark {
  width: 48px;
  height: 48px;
  display: grid;
  place-items: center;
  border-radius: 16px;
  background: linear-gradient(145deg, var(--color-primary) 0%, var(--color-primary-strong) 100%);
  border: 1px solid var(--color-stroke-strong);
  box-shadow: var(--shadow-soft);
}

.brand-mark svg {
  width: 26px;
  height: 26px;
  fill: var(--brand-icon-fill);
}

.brand-copy h1 {
  margin: 0;
  font-size: clamp(20px, 3.4vw, 26px);
  letter-spacing: -0.01em;
}

.brand-copy .tagline {
  margin: 2px 0 0;
  color: var(--color-muted);
  font-size: 14px;
}

.topbar-primary {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
}

.menu-toggle {
  appearance: none;
  border: 1px solid var(--color-border);
  background: var(--color-surface-alt);
  color: inherit;
  padding: 10px;
  border-radius: var(--radius-md);
  display: none;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  transition: transform 120ms ease, background var(--transition-base), border-color var(--transition-base), box-shadow var(--transition-base);
}

.topbar.menu-collapsible .menu-toggle {
  display: inline-flex;
}

.menu-toggle:hover {
  border-color: var(--color-stroke-strong);
  box-shadow: 0 10px 20px rgba(15, 23, 42, 0.22);
}

.menu-toggle:active { transform: translateY(1px); }

.menu-toggle-icon {
  position: relative;
  width: 20px;
  height: 14px;
  display: inline-flex;
  flex-direction: column;
  justify-content: space-between;
}

.menu-line {
  height: 2px;
  width: 100%;
  background: currentColor;
  border-radius: 999px;
  transition: transform 180ms ease, opacity 180ms ease;
}

.topbar-panel {
  display: flex;
  flex-direction: column;
  gap: 16px;
  border-top: 1px solid var(--color-border);
  padding-top: 14px;
}

.topbar.menu-collapsible .topbar-panel {
  display: none;
}

.topbar.menu-collapsible.menu-open .topbar-panel {
  display: flex;
}

.topbar.menu-open .menu-line:nth-child(1) {
  transform: translateY(6px) rotate(45deg);
}

.topbar.menu-open .menu-line:nth-child(2) {
  opacity: 0;
}

.topbar.menu-open .menu-line:nth-child(3) {
  transform: translateY(-6px) rotate(-45deg);
}

.modes {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  padding: 6px;
  border-radius: 999px;
  background: var(--color-surface-alt);
  border: 1px solid var(--color-border-soft);
  box-shadow: inset 0 1px 2px rgba(15, 23, 42, 0.12);
}

.modes .tab {
  appearance: none;
  border: 0;
  background: transparent;
  color: var(--color-muted);
  padding: 8px 16px;
  border-radius: 999px;
  font-size: 14px;
  font-weight: 600;
  letter-spacing: 0.01em;
  transition: background var(--transition-base), color var(--transition-base), box-shadow var(--transition-base);
}

.modes .tab:hover { color: var(--color-text-secondary); }

.modes .tab.active {
  background: radial-gradient(circle at top, var(--color-primary) 0%, var(--color-primary-strong) 100%);
  color: #ffffff;
  box-shadow: 0 8px 20px rgba(79, 70, 229, 0.35);
}

.topbar-actions {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  align-items: center;
}

button {
  appearance: none;
  border: 1px solid var(--color-border);
  background: var(--color-surface-alt);
  color: var(--color-text-primary);
  padding: 10px 16px;
  border-radius: var(--radius-md);
  font: inherit;
  font-weight: 600;
  cursor: pointer;
  transition: transform 120ms ease, background var(--transition-base), border-color var(--transition-base), box-shadow var(--transition-base);
}

button:hover {
  border-color: var(--color-stroke-strong);
  box-shadow: 0 10px 20px rgba(15, 23, 42, 0.22);
}

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

button.primary {
  background: linear-gradient(140deg, var(--color-primary) 0%, var(--color-primary-strong) 100%);
  border-color: transparent;
  color: #ffffff;
  box-shadow: 0 14px 30px rgba(79, 70, 229, 0.35);
}

button.secondary {
  background: var(--color-surface-alt);
}

button.secondary.active {
  border-color: var(--color-primary);
  background: var(--color-primary-soft);
  color: var(--color-text-primary);
}

.icon-button {
  width: 40px;
  height: 40px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  border-radius: 999px;
  padding: 0;
  background: var(--color-surface-alt);
  position: relative;
  overflow: hidden;
}

.icon-button .theme-icon {
  position: absolute;
  width: 18px;
  height: 18px;
  transition: opacity var(--transition-base), transform var(--transition-base);
}

.icon-button .theme-icon-sun {
  background: radial-gradient(circle, #facc15 0%, #f97316 70%);
  border-radius: 50%;
}

.icon-button .theme-icon-moon {
  background: radial-gradient(circle at 30% 30%, #e0f2fe 0%, #38bdf8 70%);
  border-radius: 50%;
  box-shadow: inset -6px -8px 16px rgba(15, 23, 42, 0.3);
  opacity: 0;
  transform: scale(0.4);
}

body:not([data-theme]),
body[data-theme='light'] {
  --theme-toggle-sun-opacity: 1;
  --theme-toggle-sun-scale: 1;
  --theme-toggle-moon-opacity: 0;
  --theme-toggle-moon-scale: 0.4;
}

body[data-theme='dark'] {
  --theme-toggle-sun-opacity: 0;
  --theme-toggle-sun-scale: 0.4;
  --theme-toggle-moon-opacity: 1;
  --theme-toggle-moon-scale: 1;
}

.icon-button .theme-icon-sun {
  opacity: var(--theme-toggle-sun-opacity, 1);
  transform: scale(var(--theme-toggle-sun-scale, 1));
}

.icon-button .theme-icon-moon {
  opacity: var(--theme-toggle-moon-opacity, 0);
  transform: scale(var(--theme-toggle-moon-scale, 0.4));
}

.container {
  max-width: 1040px;
  margin: 32px auto 48px;
  padding: 0 clamp(16px, 5vw, 40px);
  display: grid;
  gap: 24px;
}

.progress {
  color: var(--color-muted);
  font-size: 15px;
  font-weight: 600;
  letter-spacing: 0.02em;
}

.badge {
  font-size: 13px;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

.badge-success { color: var(--color-accent); }

.badge-danger { color: var(--color-danger); }

.loader,
.error {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 18px 22px;
  box-shadow: var(--shadow-strong);
}

.error p { margin: 0 0 8px; }

.file-picker {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  margin-top: 10px;
  padding: 11px 14px;
  border: 1px dashed var(--color-muted);
  border-radius: var(--radius-md);
  color: var(--color-text-primary);
  font-weight: 600;
  cursor: pointer;
  transition: border-color var(--transition-base), background var(--transition-base);
}

.file-picker:hover {
  border-color: var(--color-primary);
  background: var(--color-primary-soft);
}

.file-picker input { display: none; }

.app,
.match,
.practice {
  display: grid;
  gap: 18px;
}

.card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 28px;
  min-height: 250px;
  display: grid;
  align-items: center;
  cursor: pointer;
  transition: border-color var(--transition-base), box-shadow var(--transition-base), transform 140ms ease;
  box-shadow: var(--shadow-strong);
}

.card:hover { border-color: var(--color-stroke-strong); box-shadow: var(--shadow-elevated); }

.card:focus { outline: 2px solid var(--color-primary); outline-offset: 4px; }

.face .label {
  color: var(--color-muted);
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: 0.12em;
}

.face .text {
  margin-top: 10px;
  font-size: clamp(20px, 2.5vw, 26px);
  font-weight: 600;
  line-height: 1.5;
  white-space: pre-wrap;
  color: var(--color-text-primary);
}

.controls {
  display: flex;
  flex-wrap: wrap;
  gap: 12px;
}

/* Mix & Match */
.match-header,
.practice-header {
  display: flex;
  flex-direction: column;
  gap: 12px;
}

@media (min-width: 720px) {
  .match-header,
  .practice-header {
    flex-direction: row;
    align-items: center;
    justify-content: space-between;
  }
}

.match-actions,
.practice-actions {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
}

.match-board,
.practice-question-card,
.practice-review-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: clamp(20px, 4vw, 26px);
  box-shadow: var(--shadow-strong);
}

.match-columns {
  display: grid;
  grid-template-columns: minmax(0, 1fr) minmax(0, 1fr);
  gap: clamp(20px, 5vw, 40px);
  align-items: start;
}

@media (max-width: 900px) {
  .match-columns {
    grid-template-columns: 1fr;
  }
}

.match-divider { display: none; }

.match-questions,
.match-answers {
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.match-question,
.match-answer {
  border: 1px solid var(--color-border);
  background: var(--color-surface-alt);
  color: var(--color-text-primary);
  padding: 14px 16px;
  border-radius: var(--radius-md);
  display: grid;
  gap: 8px;
  cursor: pointer;
  transition: border-color var(--transition-base), box-shadow var(--transition-base), transform 120ms ease;
}

.match-question:hover,
.match-answer:hover { border-color: var(--color-stroke-strong); box-shadow: 0 10px 22px rgba(15, 23, 42, 0.22); }

.match-question:active,
.match-answer:active { transform: translateY(1px); }

.match-label {
  font-size: 12px;
  color: var(--color-muted);
  text-transform: uppercase;
  letter-spacing: 0.1em;
}

.match-text { font-size: 16px; line-height: 1.5; white-space: pre-wrap; }

.match-status { font-size: 13px; color: var(--color-muted); }

.match-question.active,
.match-answer.active {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 2px var(--color-primary-soft), 0 18px 35px rgba(79, 70, 229, 0.25);
}

.match-question.assigned { border-color: var(--color-primary-strong); }

.match-answer.assigned { opacity: 0.65; border-style: dashed; }

.match-question.correct {
  border-color: var(--color-accent);
  box-shadow: 0 0 0 2px rgba(16, 185, 129, 0.22);
}

.match-question.wrong {
  border-color: var(--color-danger);
  box-shadow: 0 0 0 2px rgba(248, 113, 113, 0.25);
}

.match-question.wrong .match-status { color: var(--color-danger); }

.match-result,
.practice-summary {
  text-align: center;
  color: var(--color-muted);
  min-height: 24px;
  font-size: 15px;
}

/* Practice */
.practice-list { display: grid; gap: 18px; }

.practice-question-card.missing {
  border-color: var(--color-warning);
  box-shadow: 0 0 0 2px rgba(245, 158, 11, 0.32);
}

.practice-question-card h3 {
  margin: 0;
  font-size: 18px;
  color: var(--color-text-secondary);
}

.practice-question-text {
  font-size: 17px;
  line-height: 1.6;
  white-space: pre-wrap;
}

.practice-options {
  display: grid;
  gap: 10px;
}

.practice-option {
  display: flex;
  gap: 12px;
  align-items: flex-start;
  background: var(--color-surface-alt);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 11px 14px;
  cursor: pointer;
  transition: border-color var(--transition-base), background var(--transition-base), box-shadow var(--transition-base);
}

.practice-option:hover { border-color: var(--color-stroke-strong); box-shadow: 0 8px 18px rgba(15, 23, 42, 0.2); }

.practice-option input { margin-top: 4px; accent-color: var(--color-primary); }

.practice-option span { white-space: pre-wrap; }

.practice-option.disabled {
  cursor: not-allowed;
  opacity: 0.6;
}

.practice-nav {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 12px;
}

.practice-question-counter,
.practice-review-counter {
  color: var(--color-muted);
  font-size: 14px;
  letter-spacing: 0.04em;
}

.practice-review {
  display: grid;
  gap: 14px;
}

.practice-review-options {
  list-style: none;
  margin: 0;
  padding: 0;
  display: grid;
  gap: 10px;
}

.practice-review-options li {
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 11px 14px;
  background: var(--color-surface-alt);
  display: flex;
  justify-content: space-between;
  gap: 12px;
}

.practice-review-options li.correct { border-color: var(--color-accent); }

.practice-review-options li.selected { border-color: var(--color-primary); }

.practice-review-options li.incorrect { border-color: var(--color-danger); }

.practice-review-feedback {
  font-size: 15px;
  color: var(--color-muted);
}

.practice-review-controls {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 12px;
}

.footnote {
  color: var(--color-muted);
  text-align: center;
  padding: 12px 16px 32px;
  font-size: 14px;
}

@media (min-width: 720px) {
  .face .text { font-size: clamp(22px, 2.7vw, 28px); }
}

@media (min-width: 768px) {
  .topbar {
    gap: 24px;
    grid-template-columns: minmax(0, auto) minmax(0, 1fr);
    align-items: center;
  }

  .topbar-primary {
    gap: 24px;
  }

  .menu-toggle {
    display: none;
  }

  .topbar.menu-collapsible .topbar-panel,
  .topbar-panel {
    display: flex;
    flex-direction: row;
    align-items: center;
    justify-content: space-between;
    gap: 24px;
    border-top: none;
    padding-top: 0;
    justify-self: stretch;
  }

  .modes {
    flex-wrap: nowrap;
    gap: 12px;
  }

  .topbar-actions {
    align-items: center;
  }
}

@media (max-width: 767px) {
  .topbar {
    padding: 16px;
    gap: 12px;
  }

  .brand {
    align-items: flex-start;
  }

  .brand-copy h1 {
    font-size: clamp(18px, 6vw, 22px);
  }

  .brand-mark {
    width: 44px;
    height: 44px;
  }

  .topbar-panel {
    background: var(--color-surface);
    border-radius: 16px;
    padding: 16px;
    box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.04);
  }

  .modes {
    width: 100%;
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(110px, 1fr));
    gap: 6px;
    padding: 8px;
    border-radius: 16px;
  }

  .modes .tab {
    border-radius: 12px;
    padding: 10px;
    font-size: 13px;
    text-align: center;
  }

  .topbar-actions {
    width: 100%;
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(120px, 1fr));
    gap: 10px;
  }

  .topbar-actions .icon-button {
    width: 46px;
    justify-self: center;
  }

  .controls {
    justify-content: stretch;
  }

  .controls button {
    flex: 1 1 auto;
  }
}