User blog comment:DaNASCAT/Technical Update: April 22, 2015/@comment-5961377-20150424140436

The following script no longer works... (Wikia.css) body { background :  background-attachment : fixed !important; background-size: 100 % 100 % ; }
 * 1) FFFFFF url( "imageurl" <span class="br0" style="color:rgb(0,170,0);">) <span class="kw1" style="font-weight:bold;color:rgb(133,153,0);">top center<span class="sy0" style="color:rgb(0,170,0);">;

Custom backgrounds via MediaWiki verbatim don't stretch the image to fit the screen anymore, either. Code is: body.mediawiki { background-image: url("imageurl"); }