DISCLAIMER: Patching system files using 3rd party software might be dangerous for your system. We do not recommend it and we'll not be responsible if it harms your system.
We here at AskVG love Windows customization and we post several custom 3rd party themes time to time for changing Windows look-n-feel. But Windows doesn't allow 3rd party themes by default and we need to patch Windows to be able to use those themes.
We have posted about several free utilities in past which allow you to patch Windows to use 3rd party themes but people get confused sometimes which utility should they use in which Windows version?
So today to clear all confusions and to make your life easier, we are combining all tools in this single topic.
You can use any of the following mentioned free utilities to patch your Windows and to use your favorite 3rd party theme:
NOTE: All tools support Windows XP, Vista and Windows 7.
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 Windows XP, Vista and 7.
Note that it doesn't have any UI. It'll run as a service in background and will allow you to use 3rd party themes.
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:
3. 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:
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.
You can find various themes in our following section:
Posted by: Vishal Gupta | Categories: Windows 7, Windows Vista, Windows XP
Tabrez Husain Qidwai
Thanks a lot
usman
Awesome! nice tutorial. worked perfectly
Hosein
i download a theme but it does'n have a file with .theme post fix
as you told we need a file (a.theme) and a folder with same name. so i don't have a.theme file, i just downloaded s folder contain .msstyle file. how should i do?
VG
^^ In this case you'll need to manually create a .theme file for your downloaded theme. You can follow similar steps given in following tutorial:
http://www.askvg.com/how-to-enable-hidden-aero-lite-theme-in-windows-8-rtm/
Just remember to give .theme file same name as your downloaded theme. Also while editing .theme file, set correct .msstyles file path.
Nathan Townsend
Hi. I have just installed this, works absolutely fine, but sometimes I get the classic start menu, and taskbar after turning the computer on. I do not get this normally, is this some type of bug? I don't mind it, but it kind of bugs me. Thank you for your time reading this comment!
Nathan
VG
^^ It might be a corrupt system file problem. You can try to apply default Windows theme first and then re-apply your custom theme.
Nathan Townsend
Still doesn't work.. Any idea why? Thanks VG! I'm using the theme "Windows 7 theme for Windows Vista!"
Here is the link: giannisgx89.deviantart.com/art/Windows-7-Style-For-Vista-102269037
VG
^^ I have no idea. You can try to apply a different theme and then check for a few days whether the problem still persists? If not, then it might be a problem in that particular theme.