fix dov viewer tab style

Old appearance:

![screenshot 2014-12-05 11 20 33](https://cloud.githubusercontent.com/assets/404731/5320606/c651cc16-7c70-11e4-8ebd-070c14ac7313.png)

New appearance:

![screenshot 2014-12-05 11 20 19](https://cloud.githubusercontent.com/assets/404731/5320617/cd066f26-7c70-11e4-8f5a-41c5cf93f51c.png)