You'll most likely have to make that change in the .css for the skin your using. If you look in either the Portals\_default or Portal\portal# directory you should see a folder called Skins. In that folder you should be able to find the skin you're using and under that the .css for that skin. If the same css tag is in that .css file then you need to make the change there (the style sheets cascade so it's the last one that loaded that takes precedence).