UXStyle Core is one such lite weight tool that lets you install unsupported themes for Windows 7. It’s designed for users with minimal technical knowledge. UXStyle Core tool creates a process “UnsignedThemesSvc.exe” that runs in background. Just install the software and UxStyle Core will do all necessary settings for you.This is a great way, to modify your desktop with custom themes.
You can see how to use the UxStyle Core HERE