Some extensions (guest welcome and hide control bar) have their own stylesheets. Is there a way theme writers can theme these without editing the stylesheets for the extensions? - ie some way to force the elements involved to use the code in your custom stylesheet.
One solution would be to allow each theme to have two stylesheets, one (the one we have now) applied before the extensions' sheets and one after. Then you could just add the overrides to the second one.
this thread might help you. unfortunately, with a normal vanilla installation, the theme stylesheet is loaded first, so that would have to be changed by the admin of the forum.