To enable dark or light mode in Windows 11, navigate to Settings > Personalization > Colors, then open the drop-down menu for "Choose your mode" and select "Custom". A new option will be appear. Finally set the "Choose your default Windows mode" to "Light" or "Dark".
This setting will change the look of the Windows Start menu.
To Enable Dark or Light Mode for Default Windows Mode Through Windows Registry
Warning: Modifying the Windows Registry directly through Registry Editor (Regedit) may cause problem to Windows. Proceed with caution. Before making any registry changes, make sure that you already familiar with the basics of the Windows Registry.