Board Thread:Support Requests - Designing Your Wiki/@comment-25503763-20130606211444/@comment-1158325-20130606223957

It looks like these are the MW pages for talk page message notices: Change the content of those to customize the message. For styling, first you'd need to find the ID of the popup box (it may be the same as the MW page name, but I'd have to check).
 * MediaWiki:Youhavenewmessages
 * MediaWiki:Youhavenewmessagesmulti