Change checkbox icon of selectable Tree GXT

Hi,

In my project I am using com.sencha.gxt.widget.core.client.tree.Tree to create a checkable tree.

Everything is working fine.

But now i would like to change the style of the checkboxes.

For the tree nodes I could do something like this:

tree.getStyle().setNodeOpenIcon("blabla");

Can someone tell me how I can do it for the checkboxes?

Change checkbox icon of selectable Tree GXT