(moved to the right category)
Edit: I don't understand though. The site supports automatic dark mode. The toggle in your account settings is only a manual override. You should probably double-check your browser settings.
Viewing post in DarkMode on Website without logging in
I don’t think it’s a browser problem. If there is supposed to be an auto detect for the system setting of darkmode, it’s not working because other websites are working with that feature and if it is still working I still think that it’s weird the override is only available logged in. (On my phone when I opened this convo from the Mail app with darkmode enabled, it was in light mode)