#section4 {
    position: relative;
    padding: 4rem 0;
    background-image: radial-gradient(var(--border-color) 1px, transparent 0);
    background-size: 20px 20px;
    transition: background-color 0.3s ease, color 0.3s ease;
}

#section4 .book-card {
    opacity: 1 !important;
    filter: none !important;
    transform: scale(1);
    box-shadow: 0 4px 10px rgba(0,0,0,0.1);
    transition: transform 0.3s ease, box-shadow 0.3s ease;
}

#section4 .book-card:hover {
    transform: scale(1.05);
    box-shadow: 0 8px 20px rgba(0,0,0,0.2);
}

#section4 .book-info {
    opacity: 0;
    transform: translateY(10px);
    transition: opacity 0.3s ease, transform 0.3s ease;
    pointer-events: none;
}

#section4 .book-card:hover .book-info {
    opacity: 1;
    transform: translateY(0);
    pointer-events: auto;
}

#section4 .carousel-controls {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 1rem;
}

#section4 .arrow-btn {
    border: none;
    background: none;
    font-size: 1.5rem;
    cursor: pointer;
    color: var(--text-color);
    transition: transform 0.2s ease;
}
#section4 .arrow-btn:hover {
    transform: scale(1.2);
}

#section4 .book-carousel-wrapper {
    overflow: hidden;
    width: 80%;
}

#section4 .book-carousel {
    display: flex;
    gap: 1.5rem;
    transition: transform 0.6s ease;
    flex-wrap: wrap;
    justify-content: center;
}

/* Light / Dark theme */
body.light-theme #section4,
body.light-theme #section4 * {
    --bg-color: #fff !important;
    --text-color: #222 !important;
    --border-color: #ccc !important;
    background-color: var(--bg-color) !important;
    color: var(--text-color) !important;
}

body.dark-theme #section4,
body.dark-theme #section4 * {
    --bg-color: #1e1e1e !important;
    --text-color: #eee !important;
    --border-color: #555 !important;
    background-color: var(--bg-color) !important;
    color: var(--text-color) !important;
}

/* Mobile responsive */
@media (max-width: 991px) {
    #section4 .book-carousel-wrapper {
        width: 100%;
        overflow-x: auto;
    }
    #section4 .book-card {
        flex-shrink: 0;
        width: 120px;
    }
}
@media (max-width: 479px) {
    #section4 .book-card {
        width: 100px;
    }
}
/* SECTION 4 – HOVER TRẮNG SÁNG + NÂNG CARD ĐẸP NHƯ SECTION 3 */
#section4 .book-card:hover .book-title,
#section4 .book-card:hover .book-author,
#section4 .book-card:hover small,
#section4 .book-card:hover .text-secondary {
    color: #ffffff !important;
    text-shadow: 0 2px 10px rgba(0,0,0,0.9) !important;
    transition: color 0.35s ease !important;
}

/* Nền tối mịn khi hover */
#section4 .book-cover-wrapper::after {
    content: '';
    position: absolute;
    top: 0; left: 0; right: 0; bottom: 0;
    background: rgba(0,0,0,0.72);
    border-radius: 12px;
    opacity: 0;
    transition: opacity 0.4s ease;
    pointer-events: none;
}
#section4 .book-card:hover .book-cover-wrapper::after {
    opacity: 1;
}

/* Nâng card lên nhẹ */
#section4 .book-card {
    transition: transform 0.4s ease;
}
#section4 .book-card:hover {
    transform: translateY(-12px);
}