you create an overload by defining a class with same scope, class and attribute name as current applied class, e.g. to overload
td.myclass {font-weight:bold}
you create in the skin/portal css file
td.myclass {font-weight:normal}
As those files are loaded after the default css file, the definition gets overwritten.
Check .aspx file of the skin you use for assigned cssclasses, you can also use adeveloper toolbar for your browser, to analyse classes applied to specific elements. For firefox, select toolbar on install, for IE download toolbar from http://www.microsoft.com/downloads/details.aspx?FamilyID=e59c3964-672d-4511-bb3e-2d5e1db91038&displaylang=en