Disable borders focus of customitem on qwerty

I'm working with customitem for s40 qwerty devices, however when I add my customitem in a form, and select that item, appears as a rectangle edge (I guess it's because I use CustomItem). I want to know how to disable or at least decrease the thickness of the border of this rectangle.

You will require to either use canvas directly or use some UI toolkit like LWUIT to modify the look&feel.