Remove collapse/expand arrows from TreeTable

hi guys.

Looking or help as per title. I am using the latest vaadin 7.3.x, and the Valo theme.

i want to replace the standard arrows with my own icons. I assume I have to use a cell style generator and some css for my new icons, but I need to know how to get rid of the arrows first

Any help will be greatly appreciated.

Cheers,
Lee.