Thanks to Gabriel Wicke, sysops can now add a stylesheet for their wiki using [[MediaWiki:Monobook.css]]. The content of these stylesheets should be similar to the user stylesheets. I have used this feature to increase the font size on the CJK wikis, as requested on wikitech-l. Stylesheets for the other skins and configurable javascript will be implemented soon.
-- Tim Starling