:root {
    /* Light Mode (Default) */
    --background-color: #f8f9fa;
    --surface-color: #ffffff;
    --primary-color: #6200ee;
    --on-surface-color: #333333;
    --on-background-color: #212529;
    --grid-color: rgba(0, 0, 0, 0.05);
    --shadow-color: rgba(0, 0, 0, 0.1);
}

[data-theme="dark"] {
    /* Dark Mode */
    --background-color: #121212;
    --surface-color: #1e1e1e;
    --primary-color: #bb86fc;
    --on-surface-color: #ffffff;
    --on-background-color: #ffffff;
    --grid-color: rgba(255, 255, 255, 0.05);
    --shadow-color: rgba(187, 134, 252, 0.2);
}

body {
    background-color: var(--background-color);
    color: var(--on-background-color);
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    margin: 0;
    padding: 24px;
    background-image: linear-gradient(var(--grid-color) 1px, transparent 1px), linear-gradient(90deg, var(--grid-color) 1px, transparent 1px);
    background-size: 20px 20px;
    transition: background-color 0.3s, color 0.3s;
}

header {
    text-align: center;
    margin-bottom: 48px;
    position: relative;
}

h1 {
    font-size: 2.5rem;
    color: var(--primary-color);
    text-shadow: 0 0 10px var(--shadow-color);
}

#theme-toggle {
    position: fixed;
    top: 20px;
    right: 20px;
    padding: 10px 15px;
    border-radius: 20px;
    border: 2px solid var(--primary-color);
    background-color: var(--surface-color);
    color: var(--primary-color);
    cursor: pointer;
    font-weight: bold;
    transition: all 0.3s;
    z-index: 1000;
}

#theme-toggle:hover {
    background-color: var(--primary-color);
    color: var(--surface-color);
}

#book-list {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: 24px;
    max-width: 1200px;
    margin: 0 auto 60px auto;
}

#contact {
    max-width: 600px;
    margin: 0 auto 80px auto;
    padding: 32px;
    background-color: var(--surface-color);
    border-radius: 12px;
    box-shadow: 0 10px 30px var(--shadow-color);
}

#contact h2 {
    text-align: center;
    color: var(--primary-color);
    margin-bottom: 24px;
}

.form-group {
    margin-bottom: 20px;
}

.form-group label {
    display: block;
    margin-bottom: 8px;
    font-weight: bold;
    color: var(--on-surface-color);
}

.form-group input,
.form-group textarea {
    width: 100%;
    padding: 12px;
    border: 2px solid var(--grid-color);
    border-radius: 8px;
    background-color: var(--background-color);
    color: var(--on-background-color);
    font-size: 1rem;
    box-sizing: border-box;
    transition: border-color 0.3s;
}

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

.submit-btn {
    width: 100%;
    padding: 14px;
    background-color: var(--primary-color);
    color: white;
    border: none;
    border-radius: 8px;
    font-size: 1.1rem;
    font-weight: bold;
    cursor: pointer;
    transition: opacity 0.3s, transform 0.2s;
}

.submit-btn:hover {
    opacity: 0.9;
    transform: translateY(-2px);
}

.submit-btn:active {
    transform: translateY(0);
}