fix treeview style to allow custom styling

this should solve #1444 

![2014-06-27_16h08_59](https://cloud.githubusercontent.com/assets/658431/3412816/8ffc946a-fe04-11e3-81fe-12fa915db92a.png)
![2014-06-27_16h09_12](https://cloud.githubusercontent.com/assets/658431/3412817/91c56722-fe04-11e3-97f8-6e46f9a953dc.png)

Closes #1444