Lower the margin between video thumbnails

css/_variables.scss

 $thumbnailToolbarHeight: 22px;
 $thumbnailIndicatorBorder: 2px;
 $thumbnailIndicatorSize: $thumbnailToolbarHeight;
-$thumbnailVideoMargin: 5px;
+$thumbnailVideoMargin: 2px;
 $thumbnailsBorder: 2px;
 $thumbnailVideoBorder: 2px;
 $hideFilmstripButtonWidth: 17px;