The desktop is easy to set. Simply open any image and choose "Set as desktop background" from the Image menu in the viewer. The rest of the color scheme is a bit more tricky. There are a few themes in System Settings:Appearance to choose from. You can also search for "ubuntu 12.04 themes" to find others you can install. Customizing them can be a bit painful - the files are all plain text and in /usr/share/themes. They are mostly web css files, so if you have skills there, they might be worth playing with.