If you were previously using the dark theme on this forum, you will need to re-enable it in the Interface Preferences.
We switched to a different version of the theme to make our customizations work with the latest updates. For more details, see this thread on the users forum.
That's only with Theme Material Dark & Color Scheme Dark (for some reason "Material Dark" is not always dark). Default theme + Color Scheme Dark looks like before for me.
It's not changing for me. I changed it in my preferences and saved, but it's still the default light theme. I even tried a hard refresh, but it didn't help.
The first one (Theme) allows you to switch between the default theme and the customized Material Dark theme.
Also important, though, is the second switch here. Color Scheme / Regular is used if your web browser is in "light mode" or doesn't indicate a preference, and Color Scheme / Dark is used if your browser indicates a preference for "dark mode".
If you don't want to change your device (or have a browser that doesn't report a preference), you can change the regular color scheme to dark.
Yes, it is a bit more complicated than it could be.