.wp_gallery_advanced_img100 {
    width: 100%;
    height: auto;
}

.wp_gallery_advanced_flex {
    display: -webkit-box;
    display: -moz-box;
    display: -ms-flexbox;
    display: -webkit-flex;
    display: flex;
}

.wp_gallery_advanced_thumbnails {
    flex-wrap: wrap;
}