Restyle scrollbar to match the dark theme

Before:

![screen shot 2014-03-01 at 2 41 28 am](https://f.cloud.github.com/assets/31945/2295379/477270d4-a08f-11e3-81d5-486da811183e.png)

After:

![screen shot 2014-03-01 at 2 41 54 am](https://f.cloud.github.com/assets/31945/2295384/4c09052c-a08f-11e3-8899-7b732030d6d1.png)

Colour inspired by [Phoenix Dark](https://github.com/netatoo/phoenix-theme).

:smiling_imp: