Toolbarbutton element - change edges shape

[anton kostov]

Hi,

I am developing xulrunner app that relies on toolbarbutton element as a
replacement of button, for reasons that it might be set a style for "hover".

But I don't like the rough square edges of it - how can I change them to be
oval?
Can I set via CSS or something else, for example images to be displayed at
each edge, or there is another approach?

Thanks
Anton