#overlay{
    position: fixed;
    top:0;
    left:0;
    width:100%;
    height:100%;
    background: rgba(0,0,0,0.8) none 50% / contain no-repeat;
    cursor: pointer;
    transition: 0.3s;

    visibility: hidden;
    opacity: 0;
}
#overlay.open {
    visibility: visible;
    opacity: 1;
}

#dismiss-btn { /* X button icon */
    content: "\2715";
    position: absolute;
    color:#fff;
    top: 10px;
    right:20px;
    font-size: 2em;
}

#next-slide {
    position: absolute;
    color:#fff;
    top: 320px;
    right:20px;
    font-size: 5em;
}

#preview-slide {
    position: absolute;
    color:#fff;
    top: 320px;
    left:20px;
    font-size: 5em;
}