If you're in medium trust hosting then our custom VirtualPathProvider does not work so it can't load the theme.skin file from the skin folder. In that case it uses the one from the App_Themes/Default folder so you would have to edit that one (and do it again after each upgrade since the one I ship has corner rounding enabled). In this case it will also affect all sites in an installation since they all use that same theme.skin file.
After changing it you may also need to touch the Web.config to recycle the app as the theme is heavily cached by the runtime.
Hope it helps,