MyColors is a new and very good application for Windows that will allow you to apply new themes on your system without having to deal with complicated files and configurations.
MyColors ThinkGreen doesn't comes alone, it also includes a desktop theme called ThinkGreen, a green desktop theme that will give your desktop a new and different look, like that beer advertising said: Think in green.
MyColors also offers access to lots of desktop themes to change the look of your desktop, most of them are for free. Of course if you don't like the one you are trying you can restore the original theme or choose another one. It's easy and free, with no doubt a very good choice.