Just if you have selected type of buttons = "big", now you can select the rounded border size (in pixels). By default border-radius is 2!
The left slider has buttons with border radius 100 and the other one has not border radius! Try it yourself!
Important: Note that an object embedded is necessary to handle the options of the buttons! Besides this, keep in mind that this attribute only works whether you have selected buttons type 'big', otherwise it has no effect!