Board Thread:Support Requests - Designing Your Wikia/@comment-25480045-20151213160223/@comment--20151213162730

Ahh, that would be the Chat Options then. You can follow in the steps in there to install it. You need to edit your MediaWiki:Chat.js which would involve sysop rights.

Alternatively, you can import it in your global.js page (which would appear only for you in any wikia chat) by adding in this.

importScriptPage('ChatOptions/code.js', 'dev');