WindowsBlinds 7 skins Windows. Not the way a culinary wizard peels a grape, but in replacing the visual elements with a skin. In other words, it changes the look of the interface, including the colors, windows elements such as scroll bars and tabs, fonts, etc. You don't even have to be particularly creative--WindowBlinds comes with a number of looks or "skins" that you can simply select, including a Windows 8 skin. If those don't satisfy, vendor Stardock has over 500 available at wincustomize.com.
WindowBlinds is separately downloadable element of the $50 Object Desktop, Stardock's collection of utilities that customizes Windows even further, changing not only the look of Windows, but the way it functions. WindowBlinds has a first-run tutorial that will teach you how to use the program, though it should be intuitive to anyone who's ever tweaked the Windows interface using the Appearance dialog. It has separate controls for transparency and adjusting each element of the look individually.
To create your own entire skins with their own bitmapped elements, you'll need WindowBlinds companion program--SkinStudio. SkinStudio is available separately for $20, together with WindowBlinds for $30 or as part of the aforementioned Object Desktop for the aforementioned $50. If you're a true interface tweak-head, you really should forego the piecemeal approach and buy Object Desktop.
One thing that has bugged me about Windows since Vista and now 7 arrived on the scene is the inability to change the background color in the Explorer tree and other left pane elements. With XP I always set these to a very light gray, which seems to reduce eye strain in my case. I was hoping WindowBlinds would allow this, but alas, it does not.
If you don't like the stock look of Windows or want to make it seem as if you're not really stuck back on XP (or make it look as if you were), WindowBlinds is nearly the perfect way to do it. Now if it would only change the background on that darn left Explorer panel.