Board Thread:Support Requests - Designing Your Wiki/@comment-32450965-20180614155310/@comment-9605025-20180614163201

To expand on Tupka217's response:

MediaWiki, the platform that Wikia uses, has its own markup that you use in source mode. This markup allows most in-line CSS and some HTML tags. Regardless, whatever you place on a page (whether using source mode, VisualEditor, or any of the other editors in-between) is then converted into HTML. If you simply want to change the appearance of elements on a page, you can use CSS. If you want to truely add HTML directly, you need to use JS. You can edit CSS at will but JS requires the review process. The one exception is if you are importing scripts from the FANDOM Open Source Library using MediaWiki:ImportJS.

Regardless, you must make sure to follow the customization policy that Tupka217 linked to.