#overlay {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(31, 31, 31, 0.3);
    display: none;
}

@media (max-width: 768px) {
    #overlay.active {
        display: block;
        z-index: 9;
    }
}