User blog comment:DaNASCAT/Update Your CSS and JavaScript To Speed Up Your Wiki/@comment-166269-20120831093717/@comment-3508190-20120831103511

You didn't add the MediaWiki: prefix to the CSS files from your own wiki and you've been bitten by the bug that Lunarity describes below.

This should work:

Or at least it loads fine |MediaWiki:OldInfoboxStyle.css|MediaWiki:Wikipedia.css|MediaWiki:Cardtable.css|MediaWiki:Diffs.css&only=styles on its own.