The dark background is easier on your eyes for long stints at the computer, and is also more energy efficient.
How to use
Simply download the package, copy to your Espresso themes folder and activate.
Espresso's themes folder can be easily opened while in Espresso by accessing preferences > colors and clicking 'reveal in finder' which will open Espresso's themes folder. Add the zip files content to this folder and select the theme in Espresso's preferences window to activate it.