Universal Theme Patcher

"Universal Theme Patcher" is yet another excellent UxTheme Patcher which supports Windows XP SP2/SP3, Server 2003, Vista SP1/SP2, Server 2008 and Windows 7. It can be used in both 32-bit (x86) and 64-bit (x64) systems.

You can download it using following link:
 Download Link
Following are simple steps to use this tool:
1. You just need to run its EXE file and it'll ask you to select the language.
2. Click on OK button and it'll detect the OS and condition of the system files. It'll show you the information and will ask you for confirmation.
3. Click on Yes button. Now you'll see the main interface of this tool. You just need to click on the "Patch" button given for each file:

 

4. Once you patch a file, the "Restore" button gets activated so that you can restore the default file in future.
5. Restart your system and you'll be able to use 3rd party themes in Windows without any problem.

Applying Custom 3rd Party Theme

Once you have patched Windows, copy your desired theme folder (containing a .theme file and a folder containing .msstyles file) to "%windir%\Resources\Themes\" folder (where %windir% means "Windows" directory present in system drive where Windows is installed. Most probably C: drive).

 Now double-click on .theme file and it'll apply the theme or you can apply it using Desktop Properties.