If your custom theme has the "folder-color" icons (read how to create those icons), you'll see them! By example, this is a screenshot with the awesome Numix icons (WIP yet):
|Numix icon set|
You can watch it in action in this video.
How to install: Here.
I want to thank you to Joshua Fogg from the Numix Proyect for his help & knowledge!! Really thank you ;)
Enjoy it! :)