If you want to get one of the designs you only have to add the code to either w:c:community:Special:Mypage/global.css or Special:Mypage/wikia.css. You can also click on one of the buttons to go directly to the page. If you add the code to global.css you'll see the chat skin on all the Wikia wikis when you open the chat. If you add it to wikia.css it'll only work on this wiki. Of course you can add the code or another (chat) code to other Wikia wikis on a personal wikia.css page. If you want you can also add a skin to the whole wiki. Then you can add it to MediaWiki:Wikia.css (Special:CSS). Of course don't forget to refresh after publishing your edit and then it has to work after several minutes!
Do you like it or have you found a bug please let me know! And also if you have ideas for new chat skins post a comment!