CSS tweaks for icons in the Navigation block

A small tweak to the CSS in the Navigation block.
Before:
![pull_request_before](https://f.cloud.github.com/assets/3780536/2208662/6c04fad4-9982-11e3-9c0c-fbc671659222.PNG)
After:
![pull_request_after](https://f.cloud.github.com/assets/3780536/2208665/706ce5a0-9982-11e3-8a6d-7608e2c10eb6.PNG)