I believe it is using the Vector skin (https://www.mediawiki.org/wiki/Skin:Vector)
The customization is done in Hydra.css (https://pantheonriseofthefallen.gamepedia.com/MediaWiki:Hydra.css) or custom CSS for certain elements.
For gamepedia, "Hydra" is basically their master/custom version of mediawiki, which includes additional 'features' from the default mediawiki experience.
edit: typo, wikipedia /= mediawiki, lol