Board Thread:Support Requests - Designing Your Wikia/@comment-5107036-20150313180328/@comment-5939974-20150314132539

Hey! The warning text you're looking for lies at MediaWiki:Sitenotice; since it loads on the old skins but not on Oasis (and I presume Venus, too). One thing you should note is that only logged-in users can dismiss the message. Normal readers cannot.

If you're still interested, you can follow these steps (since the source code at League of Legends Wiki is a little thick, I've modified them):
 * Create a version of Info_icon.svg using this image.
 * 1) Add this to your Mediawiki:Sitenotice:

Good luck! (This is a CSS fix, technically, so please don't harm yourself over JS frustration!)