Thread:Project:Support desk/English Wiki's pop-up page?/reply (2)

You don't need Extension:DismissableSiteNotice.

Wikipedia used Extension:CentralNotice to distribute it from the central wiki, however you don't need that extension either.

The notice system itself is in MediaWiki core. Edit on your wiki and put a the HTML for the overlay in there with CSS classes on them, and from  style them to be fixed positioned over the entire page and use   to hide everything else.