Since wiki is for community to collaborate it makes little sense to hide CSS behind admin rights. This way important part is not accessible to the community.
Idea how to implement this:
Since some may want more closed wiki, to enable this functionality, option like "Allow CSS Special Page" could be added to admin options to enable this.
-When enabled, wiki should no longer check whitelist for bbtag classes and allow everything.
-Page for CSS should be added. For example: "Special:CSS". Best would be if it was possible to have different one for each wiki area, not sure if thats possible though.
This could be just added to wiki_css.php as others are. But its cache should be rebuild automatically after "Special:CSS" is saved, unlike current CSS interface.