:root {
    --primary: #1A73E8;
    --success: #34A853;
    --warning: #FBBC04;
    --danger: #EA4335;
    --bg: #FFFFFF;
    --card-bg: #F8F9FA;
    --text-primary: #202124;
    --text-secondary: #5F6368;
    --border: #DADCE0;
    --shadow: 0 2px 8px rgba(0,0,0,0.08);
    --shadow-hover: 0 4px 16px rgba(0,0,0,0.12);
    --radius-card: 12px;
    --radius-btn: 8px;
    --transition: 0.25s ease;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Source Sans Pro', 'Noto Sans SC', sans-serif;
    background: var(--bg);
    color: var(--text-primary);
    line-height: 1.6;
    min-height: 100vh;
}

.navbar {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: 64px;
    background: var(--bg);
    box-shadow: var(--shadow);
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0 24px;
    z-index: 100;
}

.nav-brand {
    display: flex;
    align-items: center;
    gap: 12px;
}

.nav-logo {
    width: 36px;
    height: 36px;
    background: linear-gradient(135deg, var(--primary) 0%, #4285F4 100%);
    border-radius: 8px;
    display: flex;
    align-items: center;
    justify-content: center;
    color: white;
    font-weight: 700;
    font-size: 18px;
}

.nav-title {
    font-family: 'Playfair Display', serif;
    font-size: 22px;
    font-weight: 600;
    color: var(--text-primary);
}

.nav-progress {
    font-size: 14px;
    color: var(--text-secondary);
    background: var(--card-bg);
    padding: 8px 16px;
    border-radius: 20px;
}

.nav-actions {
    display: flex;
    align-items: center;
    gap: 16px;
}

.nav-btn {
    padding: 8px 16px;
    background: var(--primary);
    color: white;
    border: none;
    border-radius: var(--radius-btn);
    font-size: 14px;
    font-weight: 600;
    cursor: pointer;
    transition: background var(--transition);
}

.nav-btn:hover {
    background: #1557B0;
}

.main-content {
    padding-top: 88px;
    padding-bottom: 40px;
    max-width: 1200px;
    margin: 0 auto;
    padding-left: 24px;
    padding-right: 24px;
}

.page {
    display: none;
    animation: fadeIn 0.3s ease;
}

.page.active {
    display: block;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(10px); }
    to { opacity: 1; transform: translateY(0); }
}

.page-header {
    text-align: center;
    margin-bottom: 40px;
}

.page-title {
    font-family: 'Playfair Display', serif;
    font-size: 28px;
    font-weight: 700;
    margin-bottom: 12px;
    color: var(--text-primary);
}

.page-subtitle {
    font-size: 16px;
    color: var(--text-secondary);
}

.article-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
    gap: 24px;
}

.article-card {
    background: var(--bg);
    border-radius: var(--radius-card);
    box-shadow: var(--shadow);
    padding: 24px;
    cursor: pointer;
    transition: transform var(--transition), box-shadow var(--transition);
    border: 1px solid var(--border);
    -webkit-tap-highlight-color: transparent;
    user-select: none;
}

.article-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow-hover);
}

.article-card-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: 16px;
}

.article-title {
    font-family: 'Playfair Display', serif;
    font-size: 20px;
    font-weight: 600;
    color: var(--text-primary);
    line-height: 1.4;
}

.difficulty-badge {
    font-size: 12px;
    font-weight: 600;
    padding: 4px 12px;
    border-radius: 20px;
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.difficulty-easy {
    background: rgba(52, 168, 83, 0.1);
    color: var(--success);
}

.difficulty-medium {
    background: rgba(251, 188, 4, 0.15);
    color: #B06000;
}

.difficulty-hard {
    background: rgba(234, 67, 53, 0.1);
    color: var(--danger);
}

.article-summary {
    font-size: 15px;
    color: var(--text-secondary);
    line-height: 1.7;
    margin-bottom: 16px;
}

.article-meta {
    display: flex;
    align-items: center;
    justify-content: space-between;
    font-size: 13px;
    color: var(--text-secondary);
    padding-top: 16px;
    border-top: 1px solid var(--border);
}

.article-meta span {
    display: flex;
    align-items: center;
    gap: 4px;
}

.article-page {
    position: relative;
}

.vocabulary-page {
    position: relative;
}

.vocabulary-content {
    background: var(--bg);
    border-radius: var(--radius-card);
    box-shadow: var(--shadow);
    padding: 32px;
    border: 1px solid var(--border);
}

.vocabulary-title {
    font-family: 'Playfair Display', serif;
    font-size: 28px;
    font-weight: 700;
    margin-bottom: 24px;
    color: var(--text-primary);
    text-align: center;
}

.letter-nav {
    display: flex;
    flex-wrap: wrap;
    gap: 8px;
    justify-content: center;
    margin-bottom: 32px;
    padding-bottom: 24px;
    border-bottom: 1px solid var(--border);
}

.letter-btn {
    width: 40px;
    height: 40px;
    border: 1px solid var(--border);
    background: var(--bg);
    border-radius: var(--radius-btn);
    font-size: 14px;
    font-weight: 600;
    color: var(--text-secondary);
    cursor: pointer;
    transition: all var(--transition);
}

.letter-btn:hover {
    background: var(--card-bg);
    border-color: var(--primary);
    color: var(--primary);
}

.letter-btn.active {
    background: var(--primary);
    border-color: var(--primary);
    color: white;
}

.vocabulary-list {
    min-height: 200px;
}

.loading,
.empty {
    text-align: center;
    padding: 40px;
    color: var(--text-secondary);
    font-size: 16px;
}

.letter-section {
    animation: fadeIn 0.3s ease;
}

.letter-title {
    font-family: 'Playfair Display', serif;
    font-size: 24px;
    font-weight: 700;
    margin-bottom: 24px;
    color: var(--text-primary);
    padding-bottom: 16px;
    border-bottom: 2px solid var(--primary);
}

.words-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 16px;
}

.word-card {
    background: var(--card-bg);
    border-radius: var(--radius-btn);
    padding: 20px;
    cursor: pointer;
    transition: all var(--transition);
    border: 1px solid transparent;
}

.word-card:hover {
    background: white;
    border-color: var(--primary);
    box-shadow: var(--shadow);
    transform: translateY(-2px);
}

.word-text {
    font-family: 'Playfair Display', serif;
    font-size: 20px;
    font-weight: 600;
    color: var(--text-primary);
    margin-bottom: 8px;
}

.word-phonetic {
    font-size: 14px;
    color: var(--text-secondary);
    margin-bottom: 12px;
    font-style: italic;
}

.word-meaning {
    font-size: 15px;
    color: var(--text-primary);
    line-height: 1.6;
}

.back-btn {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    color: var(--primary);
    font-size: 15px;
    font-weight: 600;
    cursor: pointer;
    margin-bottom: 24px;
    transition: opacity var(--transition);
    background: none;
    border: none;
    padding: 8px 0;
}

.back-btn:hover {
    opacity: 0.7;
}

.back-btn svg {
    width: 20px;
    height: 20px;
}

.article-content {
    background: var(--bg);
    border-radius: var(--radius-card);
    box-shadow: var(--shadow);
    padding: 32px;
    border: 1px solid var(--border);
}

.article-content h1 {
    font-family: 'Playfair Display', serif;
    font-size: 24px;
    font-weight: 700;
    margin-bottom: 16px;
    color: var(--text-primary);
}

.article-content .meta {
    display: flex;
    align-items: center;
    gap: 16px;
    margin-bottom: 32px;
    padding-bottom: 24px;
    border-bottom: 1px solid var(--border);
}

.article-content .meta .difficulty-badge {
    font-size: 11px;
}

.article-content .meta .word-count {
    font-size: 14px;
    color: var(--text-secondary);
}

.article-text {
    font-size: 20px;
    line-height: 2.0;
    color: var(--text-primary);
}

.article-text p {
    margin-bottom: 24px;
}

.clickable-word {
    cursor: pointer;
    border-bottom: 1px dashed var(--primary);
    padding: 2px 0;
    transition: background var(--transition);
    border-radius: 2px;
}

.clickable-word:hover {
    background: rgba(26, 115, 232, 0.1);
}

.modal-overlay {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0, 0, 0, 0.5);
    display: flex;
    align-items: center;
    justify-content: center;
    z-index: 200;
    opacity: 0;
    visibility: hidden;
    transition: opacity var(--transition), visibility var(--transition);
}

.modal-overlay.active {
    opacity: 1;
    visibility: visible;
}

.modal {
    background: var(--bg);
    border-radius: var(--radius-card);
    padding: 32px;
    width: 90%;
    max-width: 400px;
    transform: scale(0.9);
    transition: transform var(--transition);
    position: relative;
}

.modal-overlay.active .modal {
    transform: scale(1);
}

.modal-close {
    position: absolute;
    top: 16px;
    right: 16px;
    width: 32px;
    height: 32px;
    border: none;
    background: var(--card-bg);
    border-radius: 50%;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: background var(--transition);
}

.modal-close:hover {
    background: var(--border);
}

.modal-word {
    font-family: 'Playfair Display', serif;
    font-size: 28px;
    font-weight: 700;
    color: var(--text-primary);
    margin-bottom: 8px;
}

.modal-phonetic {
    font-size: 16px;
    color: var(--text-secondary);
    margin-bottom: 20px;
}

.modal-meaning {
    font-size: 18px;
    color: var(--text-primary);
    line-height: 1.7;
    margin-bottom: 24px;
    padding: 16px;
    background: var(--card-bg);
    border-radius: var(--radius-btn);
}

.pronounce-btn {
    width: 48px;
    height: 48px;
    border-radius: 50%;
    background: var(--primary);
    color: white;
    border: none;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: transform var(--transition), background var(--transition);
}

.pronounce-btn:hover {
    background: #1557B0;
    transform: scale(1.05);
}

.pronounce-btn:active {
    transform: scale(0.95);
}

.pronounce-btn svg {
    width: 24px;
    height: 24px;
}

.footer {
    text-align: center;
    padding: 32px;
    color: var(--text-secondary);
    font-size: 14px;
}

.reading-controls {
    display: flex;
    align-items: center;
    gap: 16px;
    flex-wrap: wrap;
    margin-bottom: 24px;
    padding: 16px;
    background: var(--card-bg);
    border-radius: var(--radius-btn);
}

.read-btn {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 12px 24px;
    background: var(--primary);
    color: white;
    border: none;
    border-radius: var(--radius-btn);
    font-size: 16px;
    font-weight: 600;
    cursor: pointer;
    transition: background var(--transition), transform var(--transition);
}

.read-btn:hover {
    background: #1557B0;
    transform: scale(1.02);
}

.read-btn:active {
    transform: scale(0.98);
}

.read-btn svg {
    width: 20px;
    height: 20px;
}

.speed-control,
.voice-control {
    display: flex;
    align-items: center;
    gap: 8px;
}

.speed-control label,
.voice-control label {
    font-size: 14px;
    color: var(--text-secondary);
}

.speed-control select,
.voice-control select {
    padding: 8px 12px;
    border: 1px solid var(--border);
    border-radius: var(--radius-btn);
    background: var(--bg);
    font-size: 14px;
    color: var(--text-primary);
    cursor: pointer;
    outline: none;
}

.speed-control select:focus,
.voice-control select:focus {
    border-color: var(--primary);
}

.reading-progress {
    display: flex;
    align-items: center;
    gap: 8px;
    margin-left: auto;
    font-size: 14px;
    color: var(--text-secondary);
}

.reading-progress .progress-bar {
    width: 100px;
    height: 4px;
    background: var(--border);
    border-radius: 2px;
    overflow: hidden;
}

.reading-progress .progress-fill {
    height: 100%;
    background: var(--primary);
    transition: width 0.3s ease;
}

@media (max-width: 768px) {
    .navbar {
        padding: 0 16px;
    }

    .nav-title {
        font-size: 18px;
    }

    .main-content {
        padding-left: 16px;
        padding-right: 16px;
        padding-top: 80px;
    }

    .page-title {
        font-size: 24px;
    }

    .article-grid {
        grid-template-columns: 1fr;
        gap: 16px;
    }

    .article-card {
        padding: 20px 16px;
        min-height: 120px;
    }

    .article-content {
        padding: 24px 20px;
    }

    .article-content h1 {
        font-size: 22px;
    }

    .article-text {
        font-size: 18px;
    }

    .modal {
        padding: 24px;
        max-width: 320px;
    }

    .modal-word {
        font-size: 24px;
    }

    .reading-controls {
        flex-direction: column;
        align-items: stretch;
    }

    .read-btn {
        justify-content: center;
    }

    .speed-control,
    .voice-control {
        width: 100%;
    }

    .speed-control select,
    .voice-control select {
        flex: 1;
        max-width: 100%;
        overflow: hidden;
        text-overflow: ellipsis;
        white-space: nowrap;
    }

    .reading-progress {
        margin-left: 0;
        margin-top: 8px;
        justify-content: center;
    }

    .nav-actions {
        flex-direction: column;
        gap: 8px;
    }

    .nav-progress {
        font-size: 12px;
        padding: 6px 12px;
    }

    .vocabulary-content {
        padding: 20px 16px;
    }

    .vocabulary-title {
        font-size: 22px;
        margin-bottom: 16px;
    }

    .letter-nav {
        gap: 6px;
        margin-bottom: 20px;
        padding-bottom: 16px;
    }

    .letter-btn {
        width: 36px;
        height: 36px;
        font-size: 12px;
    }

    .letter-title {
        font-size: 20px;
        margin-bottom: 16px;
    }

    .words-grid {
        grid-template-columns: 1fr;
        gap: 12px;
    }

    .word-card {
        padding: 16px;
    }

    .word-text {
        font-size: 18px;
    }

    .word-phonetic {
        font-size: 13px;
    }

    .word-meaning {
        font-size: 14px;
    }
}