Match overlay border with native OS X popups

This branch uses the border color and radius from OS X popup controls in the unity overlay.

Before:
![overlay-before](https://cloud.githubusercontent.com/assets/122102/3679000/0b7a70f8-12a0-11e4-978e-97e9a19b4575.png)

After:
![overlay-after](https://cloud.githubusercontent.com/assets/122102/3678999/0b64f8b8-12a0-11e4-940a-dee32f2b7981.png)