[Mediawiki-l] Common.css

[Joris]

If i look on mediawiki.org or meta.wikimedia.org or wikipedia.org in
the page source i see:
@import "/w/index.php?title=MediaWiki:Common.css&action=raw&ctype=text/css&smaxage=2678400";

In each skin, with userstyles on or off, doesn't matter.
However i can't find how to enable this on my own wiki-website, i
think it's not on the mediawiki as distributed (i have latest version
downloaded 3 days ago).

How can i fix this?