I noticed when you change the language to certain ones in your codex preference page, the css link gets broken.
For example, try language codes like "ja" or "es" (make sure to clear cache and go to a different page), this problem can be replicated.
When I read the code, the css linked has "?" + number appended.
Though if you choose "eo" this doesn't happen.
Other mediawiki systems don't seem to have the same problem (I tried with Japanese & English WikiPedia). I don't think it's an urgent issue, but would be nice if it can be fixed.