Instead of
<link rel="stylesheet" href="http://commons.wikimedia.org/w/index.php?title=MediaWiki:Filepage.css&action=raw&maxage=2678400&usemsgcache=yes&ctype=text%2Fcss&smaxage=2678400" type="text/css" media="all" />
it should be a call to load.php of the foreign wiki loading module=filepage with only=styles and in the <head> section of the DOM ( addModuleScripts ).
This way it'll be minified and cached accordingly
See also bug 28083 (Only load it if the page exists / non-empty on the remote wiki)
Version: 1.17.x
Severity: enhancement