every rich component has a set of predefined rich-* classes. The class you've mentioned is named rich-tab-header class. Redefine it in your application and set padding to required value (CSS selectors can be used to redefine paddings not for All tab panels in your application but for concrete ones).
Refer to developer guide: http://labs.jboss.com/file-access/default/members/jbossrichfaces/freezone/docs/devguide/en/html/RichFacesComponentsLibrary.html#tabPanel to get more info on existing rich-* classes.
Thanks a ton Nick, that was teh pointer I needed !!!!