Increase contrast in navigation active items

Fixes #3451 

Not fully happy with the darker colour tone but that will do as a temporary fix.

## Before

![screen shot 2014-03-10 at 11 03 47](https://f.cloud.github.com/assets/85783/2372938/303a59c4-a844-11e3-9a72-c40006ba42fd.PNG)


## After

![screen shot 2014-03-10 at 11 03 36](https://f.cloud.github.com/assets/85783/2372945/39189600-a844-11e3-956e-f3f8d44a7da6.PNG)


![ ](http://media2.giphy.com/media/wNq7v90Qj0O8o/giphy.gif)