Prevents users from changing the look of their start menu background, such as its color or accent. By default, users can change the look of their start menu background, such as its color or accent. If you enable this setting, the user will be assigned the default start menu background and colors and will not be allowed to change them. If the "Force a specific background and accent color" policy is also set on a supported version of Windows, then those colors take precedence over this policy. If the "Force a specific Start background" policy is also set on a supported version of Windows, then that background takes precedence over this policy.
Windows Settings/
Administrative Templates/
Local Computer Policy -> Computer Configuration -> Administrative Templates -> Control Panel -> Personalization
After changing group policy options, you need to update group policy. If you do not update group policy then settings will not effect. To update it just simply type below command and also sample image is shown.
gpupdate /force
In about command we used "/force" option, this will help up to update policy options forcefully.
