Windows does not allow you to install 3rd party themes. You have to patch some system files to be able to do so. Like UxStyle Core, Universal Theme Patcher lets you install themes on Windows 7 & Vista easily.
Choose the corresponding patcher based on you Windows 32bit or 64bit and OS:
Run patch with UAC turned off. Right-click the exe file, select Run as Administrator.
XP/2003 only needs to patch one file: uxtheme.dll
2008/Vista need to patch 3 files: uxtheme.dll, themeui.dll, shsvcs.dll
Windows 7 needs to patch 3 files: uxtheme.dll, themeui.dll, themeservice.dll
Sometimes, in x64 Windows, you need to patch 2 copies of files:
Using UniversalThemePatcher-x64.exe to patch the 64bit files in \windows\system32;
And using UniversalThemePatcher-x86.exe to patch the 32bit files in \windows\syswow64.
After running patch, Restart computer to take effect.
You can run program with argument “-silent” to patch in silent mode.
You will now be able to apply 3rd party themes to change the look of your Windows 7.
You may want to check out Windows Theme Installer !