Users can normally apply it but can't revert it back when group policy is locking it down.
Open a command prompt as admin and run the following - the blue can be edited accordingly. You may need the admin password but if you're an admin needing this for helpdesk reasons that shouldn't be an issue.
rundll32.exe %SystemRoot%\system32\shell32.dll,Control_RunDLL %SystemRoot%\system32\desk.cpl desk,@Themes /Action:OpenTheme /file:"C:\Windows\Resources\Themes\aero.theme"