Since today the forum reverted to default theme in Light mode, and ignores the setting of dark mode. See below:
The UI changed places, it’s now below in the sidebar. I’m not sure why it does not take your preference though.
Moon icon is not in my sidebar. Weird. Maybe its the firefox update Ubuntu pushed to me. 
Same issue on iOS Safari 18.5, so it seems to be more widespread than that.
I reported this topic to Meta: Forum no longer honors 'dark mode' profile settings - Support - Discourse Meta
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