MediaWiki:Modern.css
From Wikipedia
Note: After saving, you may have to bypass your browser's cache to see the changes. Mozilla / Firefox / Safari: hold down Shift while clicking Reload, or press Ctrl-Shift-R (Cmd-Shift-R on Apple Mac); IE: hold Ctrl while clicking Refresh, or press Ctrl-F5; Konqueror:: simply click the Reload button, or press F5; Opera users may need to completely clear their cache in Tools→Preferences.