I came across this one this week. Did you know there are a few hidden themes built-in to Windows 7? They’re in a hidden directory, so you’ll have to type their path directly or use search with the option to location hidden files or folders.
If you navigate to C:\Windows\Globalization\MCT you’ll find several different directories. Inside of each of those folders is a directory called “Theme”. If you open that folder, you’ll see a .theme file which you can double-click to activate the theme. My favorite is the UK theme.