one additional tip, once you've made all the changes to update your skin, copy it into a new folder like yourskin_v2 then change the site to use that new version of the skin. The reason to do this is because your previous visitors have a cached copy of the CSS in their browser cache, by renaming the skin it changes the url for the css to something that is new thus forcing all visitors to get your latest css.
Hope that helps,
Joe