blue-twilight/resources/assets/css/gallery.css

.album-slideshow-container #image-preview {
    height: 600px;
    max-width: 100%;
    width: 800px;
}

.album-slideshow-container .thumbnails {
    overflow-x: scroll;
    overflow-y: hidden;
    white-space: nowrap;
    width: auto;
}