/* --- CSS Variables for Theming --- */
:root {
    --bg-color: #f4f7f9;
    --text-color: #333;
    --header-color: #2c3e50;
    --card-bg: #ffffff;
    --card-shadow: rgba(0, 0, 0, 0.1);
    --border-color: #e0e0e0;
    --btn-primary-bg: #3498db;
    --btn-primary-hover: #2980b9;
    --btn-secondary-bg: #95a5a6;
    --btn-secondary-hover: #7f8c8d;
    --tag-bg: #ecf0f1;
    --tag-text: #7f8c8d;
    --tag-active-bg: #3498db;
    --tag-active-text: #fff;
}

[data-theme="dark"] {
    --bg-color: #2c3e50;
    --text-color: #ecf0f1;
    --header-color: #ffffff;
    --card-bg: #34495e;
    --card-shadow: rgba(0, 0, 0, 0.2);
    --border-color: #4a627a;
    --btn-primary-bg: #3498db;
    --btn-primary-hover: #5dade2;
    --btn-secondary-bg: #7f8c8d;
    --btn-secondary-hover: #95a5a6;
    --tag-bg: #2c3e50;
    --tag-text: #bdc3c7;
    --tag-active-bg: #3498db;
    --tag-active-text: #fff;
}

/* --- General & Helper Classes --- */
* { box-sizing: border-box; margin: 0; padding: 0; }
body {
    font-family: 'Poppins', sans-serif;
    background-color: var(--bg-color);
    color: var(--text-color);
    line-height: 1.6;
    transition: background-color 0.3s, color 0.3s;
}
.app-container { max-width: 1200px; margin: 30px auto; padding: 20px; }
.hidden { display: none !important; }
hr { border: none; height: 1px; background-color: var(--border-color); margin: 40px 0; }
header { display: flex; justify-content: space-between; align-items: center; text-align: center; margin-bottom: 30px; }
header h1 { color: var(--header-color); flex-grow: 1; }

/* --- Buttons --- */
.btn { display: inline-block; background-color: var(--btn-primary-bg); color: #fff; padding: 10px 20px; border: none; border-radius: 5px; cursor: pointer; font-size: 0.9rem; font-weight: 600; transition: all 0.3s ease; }
.btn:hover { background-color: var(--btn-primary-hover); }
.btn:disabled { background-color: #bdc3c7; cursor: not-allowed; }
.btn-secondary { background-color: var(--btn-secondary-bg); }
.btn-secondary:hover { background-color: var(--btn-secondary-hover); }
.theme-btn { background: none; border: 1px solid var(--border-color); color: var(--text-color); font-size: 1.5rem; cursor: pointer; border-radius: 50%; width: 40px; height: 40px; line-height: 1; }

/* --- Forms & Controls --- */
.add-card-section { background-color: var(--card-bg); padding: 25px; border-radius: 8px; box-shadow: 0 4px 12px var(--card-shadow); }
.form-row { display: flex; gap: 20px; }
.form-group { margin-bottom: 20px; flex-grow: 1; }
.form-group label { display: block; font-weight: 600; margin-bottom: 5px; }
.form-group input, .form-group textarea, .collection-controls select { width: 100%; padding: 12px; border: 1px solid var(--border-color); border-radius: 5px; font-family: inherit; font-size: 1rem; background-color: var(--bg-color); color: var(--text-color); }
.collection-controls { margin-bottom: 25px; background: var(--card-bg); padding: 20px; border-radius: 8px; }
.search-sort-container { display: flex; gap: 20px; margin-bottom: 15px; }
#search-bar { width: 70%; }
.tags-container { display: flex; flex-wrap: wrap; gap: 10px; }
.tag-filter-btn { background-color: var(--tag-bg); color: var(--tag-text); border: none; padding: 5px 12px; border-radius: 15px; cursor: pointer; font-size: 0.9rem; }
.tag-filter-btn.active { background-color: var(--tag-active-bg); color: var(--tag-active-text); font-weight: 600; }

/* --- Book Collection & Flashcards --- */
.book-grid, .flashcard-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(280px, 1fr)); gap: 25px; }
.book-item { background-color: var(--card-bg); border-radius: 8px; box-shadow: 0 4px 12px var(--card-shadow); overflow: hidden; display: flex; flex-direction: column; text-align: center; }
.book-item-cover { width: 100%; height: 180px; background-color: var(--header-color); color: white; display: flex; align-items: center; justify-content: center; }
.book-item-cover img { width: 100%; height: 100%; object-fit: cover; }
.book-item-info { padding: 15px; flex-grow: 1; display: flex; flex-direction: column; }
.book-item-info h3 { margin: 0 0 10px; flex-grow: 1; }
.book-item-info p { font-size: 0.9rem; color: #7f8c8d; margin-bottom: 15px; }
.book-actions { display: flex; gap: 10px; justify-content: center; }
.flashcard { background-color: transparent; height: 220px; perspective: 1000px; cursor: pointer; position: relative; }
.flashcard .card-difficulty { position: absolute; top: 10px; left: 10px; padding: 3px 8px; font-size: 0.8rem; border-radius: 5px; font-weight: 600; z-index: 5; }
.flashcard .card-difficulty.easy { background-color: #2ecc71; color: white; }
.flashcard .card-difficulty.hard { background-color: #e74c3c; color: white; }
.card-inner { position: relative; width: 100%; height: 100%; transition: transform 0.6s; transform-style: preserve-3d; box-shadow: 0 4px 12px var(--card-shadow); border-radius: 8px; }
.flashcard.flipped .card-inner { transform: rotateY(180deg); }
.card-front, .card-back { position: absolute; width: 100%; height: 100%; -webkit-backface-visibility: hidden; backface-visibility: hidden; display: flex; flex-direction: column; padding: 20px; border-radius: 8px; overflow-y: auto; background-color: var(--card-bg); }
.card-back { transform: rotateY(180deg); }
.card-page-number { font-size: 0.8rem; font-weight: 600; color: #7f8c8d; text-align: right; }
.card-content { flex-grow: 1; }
.delete-btn { position: absolute; top: 10px; right: 10px; background: #c0392b; color: white; border: none; width: 25px; height: 25px; border-radius: 50%; cursor: pointer; font-size: 14px; line-height: 25px; text-align: center; z-index: 10; opacity: 0.7; transition: opacity 0.3s ease; }
.delete-btn:hover { opacity: 1; }

/* --- Modal & Data Management --- */
.modal-overlay { position: fixed; top: 0; left: 0; width: 100%; height: 100%; background: rgba(0, 0, 0, 0.7); display: flex; align-items: center; justify-content: center; z-index: 1000; }
.modal-content { background: transparent; padding: 20px; border-radius: 8px; width: 90%; max-width: 500px; position: relative; }
.modal-close-btn { position: absolute; top: -10px; right: -10px; background: white; border: none; border-radius: 50%; width: 30px; height: 30px; font-size: 20px; cursor: pointer; color: #333; }
#review-card-container .flashcard { height: 280px; }
.review-difficulty { display: flex; justify-content: center; gap: 20px; margin-top: 20px; }
.btn-difficulty-hard { background-color: #e74c3c; }
.btn-difficulty-easy { background-color: #2ecc71; }
.review-nav { display: flex; justify-content: space-between; align-items: center; margin-top: 20px; color: white; font-weight: 600; }
.data-management { background-color: var(--card-bg); padding: 25px; border-radius: 8px; }
.data-buttons { display: flex; gap: 15px; margin-top: 15px; }

@media (max-width: 600px) {
    .form-row, .search-sort-container { flex-direction: column; }
    #search-bar { width: 100%; }
}