.overlay {
    position: fixed;
    left: 50%;
    top: 50%;
    min-width: 340px;
    transform: translate(-50%, -50%);
    background-color: #fff;
    z-index: 1;
}

.backdrop {
    content: '';
    display: block;
    position: fixed;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, .51);
    z-index: 0;
}