I would like to report what I see as a bug, and you can try it here in the Discourse Meta site (I advise to test in an anonymous window).
If you change the theme in the hamburguer menu to another that isn’t the Minima Dark Theme, you still see the hamburguer menu with the themes, so that you can switch to another theme (or revert to the previous one) if you want.
With the Minima Dark theme, tough, it doesn’t seem possible (the other themes aren’t shown). I had to delete the cookie that store the theme id to make the theme here from meta return to the original one.
Thanks in advance!