What do you mean by "the lower part"?
The lower half of the component? Or do you mean you want the bottom edge of the button to be highlighted?
If it's the first you're going to either have to override paintComponent() to do this in the components you want or mess with look & feel a lot.
If you want the lower edge highlighted, the easiest way would be to make a new border and just set it on the components you want.