Nemo sidebar fix for treeview.

Simple fix for the Nemo sidebar gaining ugly rendering from the new
treeview. Instead of forcing the background color of the borders,
they're set transparent and pick up the correct color contextually.

Current:
![](http://i.imgur.com/TMZUz4G.png)

Proposed:
![](http://i.imgur.com/9GH2C0O.png)