Rounded corners of panel or groupbox?

Yes.  You can use the stylesheet extender to add a css class to any control. And/or use a mixin theme to set the radius style. See attached sample with both approaches.

When using CSS instead of the theme  you have to be aware of the html composition of the widget you are styling. CSS  styles need to be applied to the specifi element. For the groupbox there is an internal frame in order to have the title overlap the border. You can see the selector in the styleSheet1 extender.