Fix arrow colour on popover bottom

Before:
![before](https://f.cloud.github.com/assets/717345/2501487/bc62f8a6-b36c-11e3-85ee-20bf2dd80cf8.png)

After:
![after](https://f.cloud.github.com/assets/717345/2501492/c87264c4-b36c-11e3-9f9f-1ac09f1c32c9.png)

Stop arrow from being white on bottom popover, as the arrow sits on the title bar, so should match the title bar's background colour.