Board Thread:Support Requests - Designing Your Wikia/@comment-15664906-20140622202805/@comment-856287-20140622220931

Start here: Help:Custom chat JavaScript and CSS

These are unsupported instructions for adding custom Javascript and CSS to Wikia Chat, but they are commonly used and they do work.

That's just to allow for custom CSS though. Once you get it working, you'll still have to create a MediaWiki:Chat.css page on your wiki with the proper styles. If you need help with that, let me know what part you want to change the background on, and what sort of background you want (an image? a pattern?), and I can help you with the CSS too.