yothumbnailgallery.css

.thumbnails a { color: #000; text-decoration: none; display: inline-block; background-size: cover; background-position: center;  }
.thumbnails a:focus { outline: none; }
.thumbnails a img { border: 0; }
.thumbnails .row { text-align: center; }
.thumbnails .thumbnail { position: relative; }
.thumbnails .row .active:after { content:" "; position: absolute; top: 100%; left: 90px; border-left: 10px solid transparent; border-right: 10px solid transparent; border-top: 10px solid #000; }

.thumbnail_viewer { display: none; text-align: center; padding: 15px 65px; position: relative; }
.thumbnail_img { position: relative; max-width: 100%; background-size: contain; background-position: center; background-repeat: no-repeat; }
.thumbnail_caption { position: absolute; bottom: 0; width: 100%; padding: 1em; text-align: center; color: #fff; text-shadow: 1px 1px 1px rgba(0,0,0,.9); -moz-box-sizing: border-box; box-sizing: border-box;}
.thumbnail_left { position: absolute; top: 45%; left: 10px; width: 0; height: 0; border-top: 15px solid transparent; border-bottom: 15px solid transparent; border-right: 15px solid #000;  }
.thumbnail_right { position: absolute; top: 45%; right: 10px; width: 0; height: 0; border-top: 15px solid transparent; border-bottom: 15px solid transparent; border-left: 15px solid #000; } 
.thumbnail_close { position: absolute; top: 10px; right: 10px; font-weight: bold; font-size: 28px; }