Style 7 is a Windows 7 theme generator. It uses the default Microsoft Aero theme as base and generates a new theme using blackbox styles.
Blackbox styles are created with a single, simple, human-readable, configuration file. Thousands are also readily available for download on the internet.
You will need UxTheme Core/Patcher to run the Unsigned Themes created by Style7.
The installer includes the classic blackbox themes. But search for more themes on Customize.org
PS: I have not tried it. I believe that during installation, it may ask if you want to change your default search. Best to always create a System Restore Point first!