I just run DarkReader on my browsers, because I am a cave troll that spends far too much time staring at browser screens not to. It does a decent job with the sitepoint forums out of the box.
I don’t see the settings here, but on other Discourse forums (Including one that I know is up to date), the “Theme” setting can be found under Settings —> Preferences
I think the forum administrator needs to install a dark mode theme before the theme switcher appears.