Flat - Green is a skin for Steam. It's all on the screenshot so just take a look at it
How to enable this skin:
Download the zip file
Extract the files to your Steam\skins folder
Activate Flat - Green from Steam's options
Note: this skin uses Consolas font (MS Vista), however if you want to use different font you can easily change it. How? Go to Steam\skins\Flat - Green\Resource\
Open file called steamscheme.res in notepad. Then press CTRL+F and find Consolas, replace Consolas with desired font (for example Tahoma).
Save the file and you're done
Have fun using it.
This skin uses some of Tango Project icons.