I see, you have added a CSS rule to make all the thumbnails look the same, then all thumbnails started to look the same:

#main > .cont .thumbnails .clip .thumb-cont .thumb img {
    opacity: 1.0 !important;
    filter: none !important;
}

1. No need using "!important" here. That'd make the further modifications impossible, for e.g. the active element.
2. To highlight the active thumbnail you'll have to add a rule using the ".active" hook:

#main > .cont .thumbnails .clip .thumb-cont .thumb img {
    opacity: 1.0;
    filter: none;
}
#main > .cont .thumbnails .clip .thumb-cont .thumb.active img {
    border-bottom: 5px solid red;
}

This will add a red bottom bar to the active element.