no background-color

Cells have outer border with radius, and inner div leaks on the corner if it have a background.
Images are better to explain.

ping @damianavila

after patch
![capture decran 2013-11-26 a 17 12 46](https://f.cloud.github.com/assets/335567/1623814/b84169b2-56b8-11e3-9d1e-281efea383b5.png)

before patch
![capture decran 2013-11-26 a 17 12 27](https://f.cloud.github.com/assets/335567/1623815/b842472e-56b8-11e3-97b9-598173729c73.png)