I don't see that switch at the Late Night Discourse forum. Or at any other Discourse site I am on and have set to the Dark theme. Is it a standard Discourse thing?
I don't even know why it appears to you and not others, but it might be part of a future version of Discourse perhaps, so maybe it will arrive in due course.
I see themes listed in the menu dropdown on other forums, which makes it convenient to switch between dark/light without having to jump to the preferences page and back. For example:
This would be a nice touch if the option is available here
Knowing nothing about Discourse, I thought it may just be a setting on the back end. Now that I have looked into it a bit, seems it is featured in certain Discourse themes and/or can be added as a component. Probably not worth your time, but thanks for the offer!