Configurable widgets appearance

[redtide]

Currently Elements supports custom theme configuration globally, it would be nice to extend in some way this feature also per widget. For example I would like to customize the group background color. In this case it's possible to change the frame border color but this would also change globally all widgets frame border colors, like check boxes or radio buttons, so it would be necessary to add a frame_bgcolor but also set the custom preferences to groups only.