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

:root {
    --primary-color: #6366f1;
    --primary-dark: #4f46e5;
    --secondary-color: #8b5cf6;
    --success-color: #10b981;
    --danger-color: #ef4444;
    --text-color: #1f2937;
    --text-light: #6b7280;
    --bg-color: #ffffff;
    --bg-light: #f9fafb;
    --border-color: #e5e7eb;
    --shadow: 0 1px 3px 0 rgba(0, 0, 0, 0.1), 0 1px 2px 0 rgba(0, 0, 0, 0.06);
    --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
}

body {
    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
    color: var(--text-color);
    background-color: var(--bg-light);
    line-height: 1.6;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 20px;
}

/* Navbar */
.navbar {
    background: var(--bg-color);
    box-shadow: var(--shadow);
    padding: 1rem 0;
    position: sticky;
    top: 0;
    z-index: 100;
}

.navbar .container {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.nav-brand h1 {
    font-size: 1.5rem;
    color: var(--primary-color);
}

.nav-links {
    display: flex;
    gap: 1.5rem;
    align-items: center;
}

.nav-link {
    text-decoration: none;
    color: var(--text-color);
    font-weight: 500;
    transition: color 0.3s;
}

.nav-link:hover,
.nav-link.active {
    color: var(--primary-color);
}

.btn-logout {
    background: var(--danger-color);
    color: white;
    border: none;
    padding: 0.5rem 1rem;
    border-radius: 0.5rem;
    cursor: pointer;
    font-weight: 500;
    transition: background 0.3s;
}

.btn-logout:hover {
    background: #dc2626;
}

/* Hero Section */
.hero-section {
    background: linear-gradient(135deg, var(--primary-color) 0%, var(--secondary-color) 100%);
    color: white;
    padding: 5rem 0;
    text-align: center;
}

.hero-content {
    max-width: 800px;
    margin: 0 auto;
}

.hero-title {
    font-size: 3rem;
    margin-bottom: 1rem;
    font-weight: 700;
}

.hero-subtitle {
    font-size: 1.25rem;
    margin-bottom: 2rem;
    opacity: 0.9;
}

.hero-buttons {
    display: flex;
    gap: 1rem;
    justify-content: center;
}

/* Buttons */
.btn {
    padding: 0.75rem 1.5rem;
    border: none;
    border-radius: 0.5rem;
    font-size: 1rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.3s;
    text-decoration: none;
    display: inline-block;
}

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

.btn-primary:hover {
    background: var(--primary-dark);
    transform: translateY(-2px);
    box-shadow: var(--shadow-lg);
}

.btn-secondary {
    background: white;
    color: var(--primary-color);
}

.btn-secondary:hover {
    background: var(--bg-light);
}

.btn-block {
    width: 100%;
}

.btn-small {
    padding: 0.5rem 1rem;
    font-size: 0.875rem;
}

.btn-danger {
    background: var(--danger-color);
    color: white;
}

.btn-danger:hover {
    background: #dc2626;
}

/* Features Section */
.features-section {
    padding: 4rem 0;
    background: var(--bg-color);
}

.section-title {
    text-align: center;
    font-size: 2rem;
    margin-bottom: 3rem;
    color: var(--text-color);
}

.features-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 2rem;
}

.feature-card {
    text-align: center;
    padding: 2rem;
    background: var(--bg-light);
    border-radius: 0.5rem;
    transition: transform 0.3s;
}

.feature-card:hover {
    transform: translateY(-5px);
}

.feature-icon {
    font-size: 3rem;
    margin-bottom: 1rem;
}

.feature-card h3 {
    margin-bottom: 0.5rem;
    color: var(--text-color);
}

.feature-card p {
    color: var(--text-light);
}

/* Auth Pages */
.auth-container {
    min-height: calc(100vh - 200px);
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 2rem 0;
}

.auth-card {
    background: var(--bg-color);
    border-radius: 1rem;
    box-shadow: var(--shadow-lg);
    width: 100%;
    max-width: 450px;
    overflow: hidden;
}

.auth-tabs {
    display: flex;
    border-bottom: 2px solid var(--border-color);
}

.tab-btn {
    flex: 1;
    padding: 1rem;
    background: none;
    border: none;
    cursor: pointer;
    font-size: 1rem;
    font-weight: 500;
    color: var(--text-light);
    transition: all 0.3s;
}

.tab-btn.active {
    color: var(--primary-color);
    border-bottom: 2px solid var(--primary-color);
    margin-bottom: -2px;
}

.auth-form {
    padding: 2rem;
    display: none;
}

.auth-form.active {
    display: block;
}

.auth-form h2 {
    margin-bottom: 0.5rem;
    color: var(--text-color);
}

.auth-subtitle {
    color: var(--text-light);
    margin-bottom: 1.5rem;
}

.form-group {
    margin-bottom: 1.5rem;
}

.form-group label {
    display: block;
    margin-bottom: 0.5rem;
    color: var(--text-color);
    font-weight: 500;
}

.form-group input,
.form-group textarea,
.form-group select {
    width: 100%;
    padding: 0.75rem;
    border: 1px solid var(--border-color);
    border-radius: 0.5rem;
    font-size: 1rem;
    transition: border-color 0.3s;
}

.form-group input:focus,
.form-group textarea:focus,
.form-group select:focus {
    outline: none;
    border-color: var(--primary-color);
}

.form-row {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 1rem;
}

.error-message {
    color: var(--danger-color);
    margin-top: 1rem;
    font-size: 0.875rem;
}

.success-message {
    color: var(--success-color);
    margin-top: 1rem;
    font-size: 0.875rem;
}

/* Books Page */
.page-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin: 2rem 0;
}

.page-header h1 {
    font-size: 2rem;
    color: var(--text-color);
}

.books-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 2rem;
    margin: 2rem 0;
}

.book-card {
    background: var(--bg-color);
    border-radius: 0.5rem;
    overflow: hidden;
    box-shadow: var(--shadow);
    transition: transform 0.3s, box-shadow 0.3s;
    position: relative;
}

.book-card:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-lg);
}

.book-card.sold {
    opacity: 0.7;
}

.sold-badge {
    position: absolute;
    top: 10px;
    right: 10px;
    background: var(--danger-color);
    color: white;
    padding: 0.25rem 0.75rem;
    border-radius: 0.25rem;
    font-size: 0.75rem;
    font-weight: 600;
    z-index: 10;
}

.discount-badge {
    position: absolute;
    top: 10px;
    left: 10px;
    background: var(--success-color);
    color: white;
    padding: 0.25rem 0.75rem;
    border-radius: 0.25rem;
    font-size: 0.75rem;
    font-weight: 600;
    z-index: 10;
}

.book-image {
    width: 100%;
    height: 300px;
    background: var(--bg-light);
    display: flex;
    align-items: center;
    justify-content: center;
    overflow: hidden;
}

.book-image img {
    width: 100%;
    height: 100%;
    object-fit: cover;
}

.book-placeholder {
    font-size: 4rem;
    color: var(--text-light);
}

.book-info {
    padding: 1.5rem;
}

.book-info h3 {
    margin-bottom: 0.5rem;
    color: var(--text-color);
    font-size: 1.25rem;
}

.book-author {
    color: var(--text-light);
    margin-bottom: 0.5rem;
}

.book-condition,
.book-category {
    color: var(--text-light);
    font-size: 0.875rem;
    margin-bottom: 0.25rem;
}

.book-price {
    margin-top: 1rem;
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

.price {
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--primary-color);
}

.original-price {
    font-size: 1rem;
    color: var(--text-light);
    text-decoration: line-through;
}

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

.loading-message,
.no-books-message {
    text-align: center;
    padding: 3rem;
    color: var(--text-light);
}

/* Profile Page */
.profile-header {
    margin: 2rem 0;
}

.user-info-card {
    background: var(--bg-color);
    padding: 2rem;
    border-radius: 0.5rem;
    box-shadow: var(--shadow);
    display: flex;
    align-items: center;
    gap: 1.5rem;
    margin-top: 1rem;
}

.user-avatar {
    width: 80px;
    height: 80px;
    border-radius: 50%;
    background: var(--primary-color);
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 2.5rem;
    color: white;
}

.user-details h2 {
    margin-bottom: 0.25rem;
    color: var(--text-color);
}

.user-details p {
    color: var(--text-light);
}

.profile-tabs {
    display: flex;
    gap: 1rem;
    margin: 2rem 0;
    border-bottom: 2px solid var(--border-color);
}

.profile-tabs .tab-btn {
    padding: 0.75rem 1.5rem;
    background: none;
    border: none;
    cursor: pointer;
    font-size: 1rem;
    font-weight: 500;
    color: var(--text-light);
    border-bottom: 2px solid transparent;
    margin-bottom: -2px;
    transition: all 0.3s;
}

.profile-tabs .tab-btn.active {
    color: var(--primary-color);
    border-bottom-color: var(--primary-color);
}

/* Modal */
.modal {
    display: none;
    position: fixed;
    z-index: 1000;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.5);
    overflow: auto;
}

.modal-content {
    background-color: var(--bg-color);
    margin: 5% auto;
    padding: 2rem;
    border-radius: 0.5rem;
    width: 90%;
    max-width: 600px;
    position: relative;
    max-height: 90vh;
    overflow-y: auto;
}

.close {
    position: absolute;
    right: 1rem;
    top: 1rem;
    font-size: 2rem;
    font-weight: bold;
    color: var(--text-light);
    cursor: pointer;
    transition: color 0.3s;
}

.close:hover {
    color: var(--text-color);
}

/* Responsive */
@media (max-width: 768px) {
    .hero-title {
        font-size: 2rem;
    }

    .hero-buttons {
        flex-direction: column;
    }

    .nav-links {
        gap: 1rem;
    }

    .books-grid {
        grid-template-columns: 1fr;
    }

    .form-row {
        grid-template-columns: 1fr;
    }

    .page-header {
        flex-direction: column;
        align-items: flex-start;
        gap: 1rem;
    }
}