But if you wish to install such custom themes, you can patch the system files and remove the restrictions imposed by Microsoft. Manually replacing the system file is not advised as it may lead to system instability if done wrong.
There are many patches available online that can patch the system files with just one click of the button, but not many are available for Windows 8 just yet.
If you are looking for a patch that will work on Windows 8, then try UltraUxthemePatcher. It currently supports all Windows operating systems, starting from Windows XP to Windows 8.
|UltraUXThemePatcher Setup Window|
To patch the system files, run the downloaded file, go through the simple setup procedure.
|Patched Windows 8 RTM running Windows 8 RP Theme (w/ Aero Glass)|