Lots of parts of GitHub have dark mode but there are still quite a few, including here, the support community that are still in painful old light mode. Is there a repo where GitHub allows contributors to fix issues with their sites like this one?
In the case of the Support forum, we use Discourse to manage it.
That being said, users can change the theme in their profile preferences.
Please go to your avatar and click the profile icon, then select preferences.
From there, go to Preferences and you will see a dropdown menu for the theme.