Board Thread:Support Requests - Designing Your Wiki/@comment-30217117-20161014043113/@comment-23989026-20161014075852

It is using "class" so is retrieving the format of the categories from MediaWiki:Common.css (at the bottom of the page). It is built on this image with all 14 categories. To add to MediaWiki:Common.css requires Admin rights of the wiki.

So the starting point is what images you have and then build a template around them.