Discovered there is a link icon in bottom of the screen for any site that is opened from the Discourse Mobile app.
From there you can open the site in the mobile browser you wish.
Edit 2025-02-15
I asked one discourse site I am on if they could allow dark mode. Got a reply that it was already possible.
- Click on profile image
- Click on Profile - the icon for a person at the bottom of the list
- Click on Preferences
- Click on Interfaces
- Change Theme
- Save Changes
So I now can mostly stay in the Discourse Hub mobile app.
It works pretty well on macOS as well!