Adding CSS class attribute to the container

I think it would be nice if I can put my own CSS class to stylize the widget container. For example, I'd like to use the rounded corner for the container instead of the default flat-square one.

All the styling codes are done in CSS, part of my own theme, not part of the widget code. All you need to do is add the class='something', put the default value for 'something', and provide a field to change it in the admin screen. Not difficult task I guess.