Right now the GNOME 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.
Because respecting implicit user preferences is more accessible for site visitors with visual disabilities, you should enable this feature if at all possible.
Gitlab’s native search is abjectly terrible, and GNOME’s Gitlab instance is basically invisible to DuckDuckGo, so it’s really hard to find the correct issue tracker for a lot of things, unfortunately.
Looking at the more recent replies to the thread I linked, it sounds prefers-color-scheme might not work with all Discourse “themes”, where a “theme” can support or implement multiple color schemes in a variety of different ways.
If the option appears, but it doesn’t seem to do anything even if you enable it, there’s a Firefox extension you can use to force the prefers-color-scheme media feature to toggle to a particular value, which can be useful for testing. (Similar extensions are probably available for other browsers.)