(ui) Update collapsed navbar background color.

### Currently
On extra small devices `(max-width: 767px)`, the collapsed navbar is transparent by default.

![navbar-gray](https://cloud.githubusercontent.com/assets/2647488/5058475/278fa78e-6ca7-11e4-80ad-0930a37285d8.gif)


### Proposed:
Update the background-color of the collapsed navbar.

![navbar-black](https://cloud.githubusercontent.com/assets/2647488/5058476/299602e4-6ca7-11e4-9d0e-f3697e280e28.gif)