There are 2 methods to unblock Windows from preventing you from changing the themes. Apply the new theme from a third party.
Disable the operating system from blocking you from changing the theme.If you are not yet looking to activate the Windows operating system, but want to change its themes, you need to perform the following tasks: Closing Words How to Change Theme Without Windows 11/10 Activation.How to Change Theme Without Windows 11/10 Activation.