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/