The default checker pattern shown behind transparent images on hover is light for Hydra wikis, and dark for HydraDark wikis. If you need to force a pattern other than the default at a particular wiki, use the following code.
To dark checker pattern[]
/* Switch to dark checker background pattern for transparent images */
.filehistory a img, #file img:hover {
background: url(/skins/HydraDark/images/dark/Checker-16x16.png?3681f) repeat scroll 0 0 rgba(0,0,0,0) !important;
}
To light checker pattern[]
/* Switch to light checker background pattern for transparent images */
.filehistory a img, #file img:hover {
background: #fff url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAAAAAA6mKC9AAAAGElEQVQYV2N4DwX/oYBhgARgDJjEAAkAAEC99wFuu0VFAAAAAElFTkSuQmCC) repeat !important;
}