None, really. Adding stark (which is is probably an old word having connotations of "strong") adds emphasis to the phrase and its use would dedend on context. In addition, "naked" can be applied to inanimate objects, for instance "naked bulb", or a light globe without a shade or cover. "Stark" would only be added to "naked" when referring to people, and generally people who would normally be clothed, so it would be somewhat silly to refer to natives as being "stark naked" if they were usually unclothed unless the person describing these primitive people was utterly shocked.