:root {
  /* ── Colors ── */
  --color-text-primary:        #111;
  --color-text-secondary:      #555;
  --color-text-tertiary:       #999;
  --color-background-primary:  #fff;
  --color-background-secondary:#f5f5f5;
  --color-border-primary:      #333;
  --color-border-secondary:    #d0d0d0;
  --color-border-tertiary:     #e8e8e8;

  /* ── Badge colors ── */
  --badge-ds-bg:    #eeedfe;  --badge-ds-fg:    #3c3489;
  --badge-graph-bg: #faece7;  --badge-graph-fg: #712b13;
  --badge-dp-bg:    #faeeda;  --badge-dp-fg:    #633806;
  --badge-tree-bg:  #eaf3de;  --badge-tree-fg:  #27500a;
  --badge-sort-bg:  #e6f1fb;  --badge-sort-fg:  #0c447c;

  /* ── Typography ── */
  --font-mono: 'SF Mono', 'Consolas', monospace;
  --font-size-xs:   11px;
  --font-size-sm:   12px;
  --font-size-md:   13px;
  --font-size-base: 14px;
  --font-size-lg:   15px;
  --font-weight-normal: 400;
  --font-weight-medium: 500;

  /* ── Spacing ── */
  --space-xs:  4px;
  --space-sm:  6px;
  --space-md:  8px;
  --space-lg:  10px;
  --space-xl:  12px;
  --space-2xl: 14px;
  --space-3xl: 16px;
  --space-4xl: 1rem;
  --space-5xl: 1.5rem;
  --space-6xl: 1.8rem;
  --space-7xl: 2rem;

  /* ── Border radius ── */
  --radius-sm: 6px;
  --radius-md: 8px;
  --radius-lg: 12px;
  --radius-pill: 20px;

  /* ── Transitions ── */
  --transition-fast:   0.12s;
  --transition-normal: 0.15s;
  --transition-slow:   0.2s;

  /* ── Layout ── */
  --grid-min-col:  290px;
  --grid-gap:      var(--space-xl);
  --card-padding:  var(--space-4xl) 1.1rem;
  --line-height:   1.6;

  /* ── System Preferences ── */
  color-scheme: light dark;
}

/* ── Dark mode (data-theme) ── */
:root[data-theme="dark"] {
  /* Base colors */
  --color-text-primary:        #cccccc;
  --color-text-secondary:      #a0a0a0;
  --color-text-tertiary:       #808080;
  --color-background-primary:  #222222; 
  --color-background-secondary:#2a2a2a; 
  --color-border-primary:      #4d4d4d;
  --color-border-secondary:    #3c3c3c;
  --color-border-tertiary:     #2d2d2d;

  /* Badges */
  --badge-ds-bg:    #2a2854;  --badge-ds-fg:    #b8b3f0;
  --badge-graph-bg: #40261d;  --badge-graph-fg: #f0a88a;
  --badge-dp-bg:    #403114;  --badge-dp-fg:    #edc478;
  --badge-tree-bg:  #213810;  --badge-tree-fg:  #a8d47a;
  --badge-sort-bg:  #163252;  --badge-sort-fg:  #7ab8e8;
}

/* ── Reset (scoped) ── */
html, body {
  background-color: var(--color-background-primary);
  color: var(--color-text-primary);
}

.wrap,
.wrap * {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  -webkit-tap-highlight-color: transparent;
}

.wrap {
  padding: var(--space-md) var(--space-4xl) var(--space-4xl);
  scroll-behavior: smooth;
}

/* ── App Header ── */
.app-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: var(--space-4xl);
}

.app-title {
  font-size: 1.5rem;
  font-weight: 600;
  color: var(--color-text-primary);
}

/* ── Theme Toggle ── */
.theme-toggle {
  background: transparent;
  border: none;
  font-size: 1.25rem;
  cursor: pointer;
  padding: var(--space-sm);
  border-radius: var(--radius-md);
  transition: background var(--transition-normal);
  color: var(--color-text-primary);
  display: flex;
  align-items: center;
  justify-content: center;
  touch-action: manipulation;
  min-height: var(--tap-target-min);
  min-width: var(--tap-target-min);
  -webkit-tap-highlight-color: transparent;
}

.theme-toggle:hover {
  background: var(--color-background-secondary);
}

.theme-toggle:focus-visible {
  outline: 2px solid var(--color-border-primary);
  outline-offset: 2px;
}

.icon-sun { display: none; }
.icon-moon { display: block; }

:root[data-theme="dark"] .icon-sun { display: block; }
:root[data-theme="dark"] .icon-moon { display: none; }

/* ── Tabs ── */
.tabs {
  display: flex;
  gap: var(--space-md);
  flex-wrap: wrap;
  margin-bottom: var(--space-5xl);
}

.tab {
  padding: var(--space-md) var(--space-3xl);
  min-height: var(--tap-target-min);
  border-radius: var(--radius-pill);
  border: 0.5px solid var(--color-border-secondary);
  background: transparent;
  cursor: pointer;
  font-size: var(--font-size-md);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-secondary);
  touch-action: manipulation;
  transition: background var(--transition-normal),
              color var(--transition-normal),
              border-color var(--transition-normal);
}

.tab:active {
  opacity: 0.7;
}

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

.tab:focus-visible {
  outline: 2px solid var(--color-border-primary);
  outline-offset: 2px;
}

.tab.active {
  background: var(--color-text-primary);
  color: var(--color-background-primary);
  border-color: var(--color-text-primary);
}

/* ── Search ── */
.wrap search {
  display: block;
  margin-bottom: var(--space-6xl);
}

.search-input {
  width: 100%;
  padding: var(--space-lg) var(--space-2xl);
  min-height: var(--tap-target-min);
  border: 0.5px solid var(--color-border-secondary);
  border-radius: var(--radius-md);
  font-size: 16px; /* prevents iOS zoom on focus */
  background: var(--color-background-primary);
  color: var(--color-text-primary);
  transition: border-color var(--transition-normal);
}

.search-input:focus {
  outline: none;
  border-color: var(--color-border-primary);
}

/* ── Grid ── */
.grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(var(--grid-min-col), 1fr));
  gap: var(--grid-gap);
  align-items: start;
}

/* ── Card ── */
.card {
  background: var(--color-background-primary);
  border: 0.5px solid var(--color-border-tertiary);
  border-radius: var(--radius-lg);
  padding: var(--card-padding);
  cursor: pointer;
  touch-action: manipulation;
  transition: border-color var(--transition-normal);
}

.card:active {
  opacity: 0.85;
}

.card:hover {
  border-color: var(--color-border-primary);
}

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

.card.open {
  border-color: var(--color-border-primary);
  grid-column: 1 / -1;
}

.card-head {
  display: flex;
  align-items: center;
  gap: var(--space-lg);
}

/* ── Badges ── */
.badge {
  font-size: var(--font-size-xs);
  font-weight: var(--font-weight-medium);
  padding: 3px 9px;
  border-radius: var(--radius-pill);
  white-space: nowrap;
}

.badge-ds    { background: var(--badge-ds-bg);    color: var(--badge-ds-fg); }
.badge-graph { background: var(--badge-graph-bg); color: var(--badge-graph-fg); }
.badge-dp    { background: var(--badge-dp-bg);    color: var(--badge-dp-fg); }
.badge-tree  { background: var(--badge-tree-bg);  color: var(--badge-tree-fg); }
.badge-sort  { background: var(--badge-sort-bg);  color: var(--badge-sort-fg); }

.card-title {
  font-size: var(--font-size-lg);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-primary);
  flex: 1;
}

.chevron {
  font-size: var(--font-size-sm);
  color: var(--color-text-tertiary);
  transition: transform var(--transition-slow);
}

.card.open .chevron {
  transform: rotate(90deg);
}

.card-body {
  display: grid;
  grid-template-rows: 0fr;
  opacity: 0;
  visibility: hidden;
  margin-top: 0;
  border-top: 0.5px solid transparent;
  transition: grid-template-rows 0.3s cubic-bezier(0.4, 0, 0.2, 1),
              margin-top 0.3s cubic-bezier(0.4, 0, 0.2, 1),
              border-color 0.3s cubic-bezier(0.4, 0, 0.2, 1),
              opacity 0.2s ease-out,
              visibility 0.3s;
}

.card.open .card-body {
  grid-template-rows: 1fr;
  opacity: 1;
  visibility: visible;
  margin-top: var(--space-xl);
  border-top-color: var(--color-border-tertiary);
}

.card-inner {
  overflow: hidden;
  min-height: 0;
  padding-top: var(--space-xl);
}

.section-label {
  font-size: var(--font-size-xs);
  font-weight: var(--font-weight-medium);
  color: var(--color-text-tertiary);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin: var(--space-lg) 0 var(--space-xs);
}

.desc {
  font-size: var(--font-size-md);
  color: var(--color-text-secondary);
  line-height: var(--line-height);
}

/* ── Operations (<dl>) ── */
.ops {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-xs);
  margin-top: var(--space-xs);
}

.op-item {
  display: flex;
  justify-content: space-between;
  padding: var(--space-xs) var(--space-md);
  font-size: var(--font-size-sm);
  background: var(--color-background-secondary);
  border-radius: var(--radius-sm);
}

.op-name {
  color: var(--color-text-secondary);
}

.op-tc {
  font-weight: var(--font-weight-medium);
  font-size: var(--font-size-xs);
  font-family: var(--font-mono);
  color: var(--color-text-primary);
}

/* ── Use-case tags (<ul>/<li>) ── */
.uses-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-xs);
  margin-top: var(--space-xs);
  list-style: none;
}

.use-tag {
  font-size: var(--font-size-xs);
  padding: var(--space-sm) var(--space-md);
  min-height: 32px;
  display: flex;
  align-items: center;
  background: var(--color-background-secondary);
  border-radius: var(--radius-sm);
  color: var(--color-text-secondary);
  line-height: 1.4;
  cursor: pointer;
  touch-action: manipulation;
  border: 0.5px solid transparent;
  transition: border-color var(--transition-fast),
              color var(--transition-fast);
}

.use-tag:hover {
  border-color: var(--color-border-secondary);
  color: var(--color-text-primary);
}

.use-tag:active {
  opacity: 0.7;
}

.use-tag:focus-visible {
  outline: 2px solid var(--color-border-primary);
  outline-offset: 1px;
}

/* ── Ask button ── */
.ask-btn {
  margin-top: var(--space-xl);
  padding: var(--space-md) var(--space-2xl);
  min-height: var(--tap-target-min);
  border: 0.5px solid var(--color-border-secondary);
  border-radius: var(--radius-md);
  background: transparent;
  font-size: var(--font-size-sm);
  cursor: pointer;
  color: var(--color-text-secondary);
  touch-action: manipulation;
  transition: background var(--transition-normal);
}

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

.ask-btn:active {
  opacity: 0.7;
}

.ask-btn:focus-visible {
  outline: 2px solid var(--color-border-primary);
  outline-offset: 2px;
}

/* ── Empty state ── */
.empty {
  text-align: center;
  padding: var(--space-7xl);
  color: var(--color-text-tertiary);
  font-size: var(--font-size-base);
  grid-column: 1 / -1;
}

/* ── Footer ── */
.app-footer {
  text-align: center;
  margin-top: var(--space-5xl);
  padding-top: var(--space-3xl);
  font-size: var(--font-size-sm);
  color: var(--color-text-tertiary);
}

.app-footer a {
  color: var(--color-text-secondary);
  text-decoration: none;
  font-weight: var(--font-weight-medium);
  transition: color var(--transition-fast);
}

.app-footer a:hover {
  text-decoration: underline;
  color: var(--color-text-primary);
}

/* ── Mobile responsive ── */
@media (max-width: 640px) {
  :root {
    --grid-min-col: 100%;
  }

  .wrap {
    padding: var(--space-4xl) var(--space-3xl);
  }

  .tab {
    padding: var(--space-md) var(--space-2xl);
    font-size: var(--font-size-base);
  }

  .ops { grid-template-columns: 1fr; }
  .uses-grid { grid-template-columns: 1fr; }
}