Change Windows 10 & 11 Theme Without Activating Windows
While you can use Windows 10 without activating it, there are some limitations, most notably that you can't change the wallpaper or theme.
When you try to personalize Windows 10, you will get a message that says, "You need to activate Windows before you can personalize your PC."
Before you get started, you will need to find a themepack for Windows 10.
We need to make some changes to the Group Policy Editor or the Registry Editor. Windows 10 Home users must use the registry or enable Group Policy Editor.
Option 1: Change Windows 10 Theme Without Activating Windows - Group Policy Editor