I have a visual disability that leads me to use Dark Mode 24/7, and Dark Mode software support is a major focus area for me.
All modern web browsers support the prefers-color-scheme CSS media feature for automatically choosing between Light Mode and Dark Mode CSS styles.
Right now the Nextcloud Discourse requires users to create an account, log in, and then go in their site preferences in order to enable the Discourse Dark Mode CSS.
However, it seems like Discourse in particular supprts enabling prefers-color-scheme as the default, while still allowing users to optionally override it.
Because respecting implicit user preferences is more accessible for site visitors with visual disabilities, you should enable this feature if at all possible.
I posted about this on the GNOME Discourse a couple days ago, and they seem to have been able to set this up there.