To disable the blurring the of the background windows with this DWORD registry value:
After having changed the registry value, then change the theme to Basic (notice blur is gone), then change to the wanted theme (the blur remains gone).
[HKEY_CURRENT_USER \Software \Microsoft \Windows \DWM]
ColorizationGlassReflectionIntensity = 0 (0 = No blur, Default = 50)
Credits Tweaking with Vishal