installing 3rd-party themes on windows

 First you need to create a restore point.If you don't know how to create a restore point know how here To create a restore point

You can use any of the following mentioned free utilities to patch your Windows and to use your favorite 3rd party theme:

1. UxStyle

"UxStyle" is a very small lightweight utility created by our friend "Rafael" to use 3rd party themes in Windows. According to the developer:

UxStyle is a light-weight system service named Unsigned Themes, complimentary to the Themes service, and a kernel driver, sizing in at ~500k and ~17kb respectfully (beta builds). The service handles the enabling/disabling of custom theme support and the kernel driver handles patching. For 64-bit platforms, the kernel driver is signed with a digital certificate, as required by Microsoft.

Simply download and install the tool and get ready to enjoy 3rd party themes. It works in almost all Windows versions including Windows XP, Vista, Windows 7, Windows 8 and Windows 8.1. It also supports Windows Server 2003, 2008, 2008 R2, 2012 and 2012 R2.
It'll run as a service in background and will allow you to use 3rd party themes.
Download Link

2. VistaGlazz

"VistaGlazz" is another free and very easy to use UXtheme patcher for Windows XP, Vista and 7.

Download and run the program, accept the agreement and click on the first "Computer" icon to patch your system.
You can download it using following link:
Download Link

3. Universal Theme Patcher

"Universal Theme Patcher" is yet another excellent UxTheme Patcher which supports Windows XP, Server 2003, Vista, 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.