I noticed something since my last post, so to qualify - when logged out, I noticed discourse picks up the OS theme as dark, sets the discourse site as dark in tandem and thus the loading screen uses dark background color also to sync with theme-styling
However , when logged in as a user that is enabled dark theme as default, it reverts to white loading screen.
So I might be right in saying it s already 50% working as expected. 
I haven’t tested it out maybe someone else might give it a try.