* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: Arial, sans-serif;
  background-color: #121212;
  color: #e5e5e5;
}

.app-header {
  text-align: center;
  padding: 2rem 1rem;
  background-color: #1a1a1a;
  border-bottom: 1px solid #333;
}

.app-header h1 {
  margin: 0;
  color: #c084fc;
}

.app-header p {
  color: #aaa;
}

.app-nav {
  margin-top: 1rem;
}

button {
  padding: 0.65rem 1rem;
  border: none;
  border-radius: 8px;
  background-color: #333;
  color: white;
  cursor: pointer;
}

button:hover {
  filter: brightness(1.15);
}

.app-nav button {
  margin: 0.25rem;
}

.app-container {
  width: min(1100px, 92%);
  margin: 2rem auto;
}

.app-section {
  margin-bottom: 2rem;
  padding: 1.5rem;
  background-color: #1b1b1b;
  border: 1px solid #333;
  border-radius: 14px;
}

.hidden {
  display: none;
}

form {
  display: grid;
  gap: 0.75rem;
}

input,
textarea,
select {
  width: 100%;
  padding: 0.75rem;
  border-radius: 8px;
  border: 1px solid #444;
  background-color: #101010;
  color: #eee;
}

textarea {
  min-height: 90px;
  resize: vertical;
}

.message {
  color: #c084fc;
  min-height: 1.2rem;
}

.library-controls {
  display: grid;
  grid-template-columns: 1fr 220px;
  gap: 1rem;
  margin-bottom: 1rem;
}

.card-list {
  display: grid;
  gap: 1rem;
}

.library-card {
  padding: 1rem;
  background-color: #242424;
  border: 1px solid #3a3a3a;
  border-radius: 12px;
}

.library-card h3 {
  margin-top: 0;
  color: #c084fc;
}

.card-actions {
  display: flex;
  gap: 0.5rem;
  margin-top: 1rem;
}

.delete-btn {
  background-color: #9f1239;
}

.edit-btn {
  background-color: #4c1d95;
}

.study-panel {
  margin-top: 1rem;
  padding: 1rem;
  background-color: #202020;
  border: 1px solid #333;
  border-radius: 12px;
}

.study-card {
  perspective: 1000px;
  max-width: 650px;
  margin: 2rem auto;
}

.card-inner {
  position: relative;
  width: 100%;
  height: 260px;
  transition: transform 0.4s ease;
  transform-style: preserve-3d;
}

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

.card-front,
.card-back {
  position: absolute;
  width: 100%;
  height: 100%;
  padding: 2rem;
  backface-visibility: hidden;
  border-radius: 14px;
  background-color: #181818;
  border: 1px solid #444;
  box-shadow: 0 0 20px rgba(0, 0, 0, 0.6);
  text-align: center;
  display: grid;
  place-content: center;
}

.card-front h3,
.card-back h3 {
  color: #888;
  text-transform: uppercase;
  letter-spacing: 1px;
}

.card-front p,
.card-back p {
  font-size: 1.4rem;
  line-height: 1.4;
}

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

.study-actions {
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
  gap: 0.75rem;
}

#study-setup {
  display: flex;
  flex-direction: column;
  gap: 1rem;
  max-width: 1050px;
}

#correct-btn {
  background-color: #166534;
}

#incorrect-btn {
  background-color: #991b1b;
}

#flip-card-btn {
  background-color: #4c1d95;
}

#review-missed-btn {
  background-color: #7e22ce;
}

.app-footer {
  text-align: center;
  padding: 1rem;
  color: #777;
}

@media (max-width: 700px) {
  .library-controls {
    grid-template-columns: 1fr;
  }

  .card-inner {
    height: 320px;
  }
}