we enabled dark mode a couple of years ago. last year, i added a theme component that let users enable automatic light/dark mode theme switching, based on their device settings.
that component is now deprecated, so i removed it. automatic light/dark theme switching is now built into Discourse core, as of 2.6.0beta2 (our current version).
logged-in users can still disable the automated theme switching, as well as statically set the light or dark theme across all their devices. go to Profile > Preferences > Interface.
bonus: non-logged-in users are also no longer greeted by blinding brightness when browsing late at night, if they’re using a dark theme on their device. the forum’s theme now switches to match their device’s theme, whether dark or light.