:root {
  --bg: #ffffff;
  --fg: #222;
  --accent: #3b82f6;
  --border: #ddd;
  --radius: 6px;
  --sidebar-w: 240px;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #1e1e1e;
    --fg: #e5e5e5;
    --border: #333;
  }
}

body {
  margin: 0;
  background: var(--bg);
  color: var(--fg);
  font-family: system-ui, sans-serif;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

.app-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: .75rem 1rem;
  border-bottom: 1px solid var(--border);
}

.layout {
  display: grid;
  grid-template-columns: var(--sidebar-w) 1fr;
  flex: 1;
}

.sidebar {
  padding: 1rem;
  border-right: 1px solid var(--border);
  overflow-y: auto;
}

.deck-list {
  margin: 0;
  padding: 0;
  list-style: none;
}

.deck-list li {
  padding: .5rem;
  border-radius: var(--radius);
  cursor: pointer;
}

.deck-list li.active {
  background: var(--accent);
  color: white;
}

.main {
  padding: 1rem;
  overflow-y: auto;
}

.btn {
  background: var(--accent);
  color: white;
  border: none;
  padding: .5rem 1rem;
  border-radius: var(--radius);
  cursor: pointer;
}

.btn:focus {
  outline: 3px solid #2563eb;
}

.input {
  padding: .5rem;
  border: 1px solid var(--border);
  border-radius: var(--radius);
}

.study-card {
  width: 100%;
  max-width: 420px;
  height: 260px;
  margin: 1rem auto;
  position: relative;
  perspective: 1000px;
  cursor: pointer;
  transform-style: preserve-3d;
}

.card-face {
  position: absolute;
  inset: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  font-size: 1.25rem;
  padding: 1rem;
  border-radius: var(--radius);
  border: 1px solid var(--border);
  backface-visibility: hidden; 
  background: white;
  transition: transform 0.6s ease;
}

.card-back {
  transform: rotateY(180deg);
}

.study-card.flipped .card-front {
  transform: rotateY(180deg);
}

.study-card.flipped .card-back {
  transform: rotateY(0deg);
}

.card-list .card-item {
  padding: .75rem;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  margin-bottom: .5rem;
  background: white;
}

.card-item-header {
  display: flex;
  justify-content: space-between;
}

.card-item-actions button {
  margin-left: .5rem;
}

.modal {
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,.4);
  display: flex;
  align-items: center;
  justify-content: center;
}

.modal-content {
  background: var(--bg);
  padding: 1rem;
  border-radius: var(--radius);
  width: 90%;
  max-width: 340px;
  /* display: flex; */
  /* align-items: center; */
  /* justify-content: center; */
}

.hidden {
  display: none;
}

@media (max-width: 600px) {
  .layout {
    grid-template-columns: 1fr;
    grid-template-rows: auto auto 1fr;
  }
}