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

body {
    font-family: system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
    background: #f4f6fb;
    color: #222;
    min-height: 100vh;
    padding: 40px 20px;
}

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

header h1 {
    font-size: 2.4rem;
    color: #1a237e;
}

header p {
    color: #666;
    margin-top: 8px;
}

.books-grid {
    max-width: 900px;
    margin: 0 auto;
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));
    gap: 16px;
}

.btn {
    display: block;
    padding: 18px 24px;
    background: #3f51b5;
    color: #fff;
    text-decoration: none;
    border-radius: 10px;
    text-align: center;
    font-weight: 600;
    font-size: 1.05rem;
    box-shadow: 0 3px 8px rgba(63, 81, 181, 0.25);
    transition: transform .15s, background .15s, box-shadow .15s;
}

.btn:hover {
    background: #303f9f;
    transform: translateY(-3px);
    box-shadow: 0 6px 16px rgba(63, 81, 181, 0.35);
}

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

.loading, .empty, .error {
    grid-column: 1 / -1;
    text-align: center;
    color: #666;
    padding: 20px;
}

.error { color: #c62828; }