Update: Note: Windows 7 SP1 users CLICK HERE!.
You need to patch uxtheme.dll, themeui.dll, themeservice.dll for using the third party visual styles in Windows 7. I made uxtheme patch threads for Windows Vista and XP users but couldn't make the same for Windows 7 because Rafael, the man behind the patched files decided to kill the future of the patched uxtheme.dll files and released a free Uxtheme memory patcher (UxStyle Core) for all the versions of Window, including Windows 7 (which actually is good).
The problem is that many Windows users don't want to see additional process running on their Task Manager from Rafael's UxStyle Core. That's why I'm sharing Uxtheme Multi Patcher for Windows 7 here which will patch all the above mentioned dll files for applying the custom visual styles on Windows 7.
Uxtheme Multi Patcher for Windows 7 (Automatic Process)
Here is the easy way for those who aren't skilled enough to replace system files in Windows 7. 
. Just run the patcher, restart your PC and you are done!
Download: Uxtheme_Patch_for_Windows_7.zip | Mirror (for both 32 bit and 64 bit of Windows 7)
Note: This patch is for non-SP1 user. If you don't know whether you have SP1 installed or not then most probably you have SP1 installed. Go to this page for downloading the updated patch which is compatible with Windows 7 SP1.
The Multi Patcher requires administrator rights. Right-click the exe file, select "Run as Administrator" and then restart computer before applying the visual styles.
You are done! 
Download Windows 7 themes from here and enjoy the new look.
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For Expert Windows Users:
If you already have the patched uxtheme.dll, themeui.dll, themeservice.dll files and don't want to run any patcher then use my this guide to replace them with the original files.
How to replace the patched files manually?
- Login as a user with Administrative privileges
- Navigate to %systemroot%\System32
- Right-click uxtheme.dll and click properties.
- Take ownership of the file and close dialogs.
- Repeat steps 3 and 4 for themeui.dll and themeservice.dll
- Right-click uxtheme.dll and click properties.
- Give yourself (or group) Full Control permissions and close dialogs.
- Repeat steps 6 and 7 for themeui.dll and themeservice.dll.
- Rename uxtheme.dll to uxtheme.old.
- Rename themeui.dll to themeui.old.
- Rename themeservice.dll to themeservice.old.
- Copy patched files into %systemroot%\System32
- Reboot (yes, this is really required)
- Enjoy!
If you can't follow these simple steps then use Take Control for taking ownership of system files before replacing them.