Styles are currently loaded in improper order, where user's style is not loaded as last (concretely eg. GeSHi syntax highlight styles are loaded after).
The proper order should be:
Built-in styles (such as /skins/.../main.css)
MediaWiki:Common.css
MediaWiki:Skin.css
MediaWiki:Extension(s).css (such as syntax highlighting etc.)
User:Common.css (see bug 10183 for details)
User:Skin.css
Version: unspecified
Severity: trivial