Open regedit.exe,
go to Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Themes\Personalize and set both AppsUseLightTheme & SystemUsesLightTheme DWORDs to 0.
Alternatively, download and save the windows-dark-mode.reg file below as a .reg file and run it.
Source: https://old.reddit.com/r/Windows11/comments/o1l55u/how_do_i_enable_dark_mode_on_windows_11/h21f69b/
Windows 11 Home unlicensed limits the ability to personalize appearance, including dark mode. This is a workaround. There are some people (including myself) who can't/won't fork up the money.