style popup scrollbar

From:
![screen shot 2014-09-10 at 6 49 53 pm](https://cloud.githubusercontent.com/assets/108094/4226922/d357d300-393c-11e4-9bf3-50f7592bb015.png)

To:
![screen shot 2014-09-10 at 6 49 34 pm](https://cloud.githubusercontent.com/assets/108094/4226925/d7cf4526-393c-11e4-939a-37a3b1b6dd8d.png)