vertical tab too depended from theme_fieldset

In my theme I hooked theme_fieldset because <fieldset><legend /></fieldset> was not styleable as reqested. i canged it to an output like <fieldset><div.legend></fieldset>. Unfortunately it influenced horizontal tabs (not the core vertical tabs btw) in a wrong way: The tab gets unlabeled.

considering horizontal tabs, after all, look totally different to a fieldset there shouldn't be a dependency on the theme layer, I think. vertical tabs should maybe theme its own fieldsets.