Fix up secondary nav dropdowns when wallpapers are enabled

I wanted to avoid throwing z-indexes at z-indexes. As far as I could see it was because of this issue with the bottom shroud:

![image](https://cloud.githubusercontent.com/assets/814861/4060608/4600834a-2df1-11e4-85de-1c214a983113.png)

But if I remove the z-index from the pseudo element it seems to be all good:

![image](https://cloud.githubusercontent.com/assets/814861/4060655/a36e305e-2df1-11e4-8fe3-6e591d50621b.png)

Already checked it in browsers but will do so again