Updated tree-view.less to include styles for scrollbars.

Updated tree-view.less to include styles for scrollbars.

The theme was initially showing standard scrollbars (white) so I added a few lines of CSS to customize the look.

I used the colours of the theme so that everything blends in.


![screen shot 2014-03-12 at 4 33 01 pm](https://f.cloud.github.com/assets/909749/2393602/0ccf42de-a997-11e3-8aa6-9b4b02a1c65d.png)