Improve contrast for selected item in overlay list

In my opinion it's currently very hard to see which item is selected in the overlay (the ones that ⌘-P/⌘-T open), so I added a background color which makes it way easier to see it.

Before:
![overlays_less_-__users_rmehner__atom_packages_atom-soda-dark-ui](https://f.cloud.github.com/assets/82050/2325083/9611d63c-a3d0-11e3-9f4c-27a29ffb5133.png)

After:
![overlays_less_-__users_rmehner__atom_packages_atom-soda-dark-ui](https://f.cloud.github.com/assets/82050/2325095/c195e910-a3d0-11e3-88e9-3982b7bb3aeb.png)