To the administrators of the forum. Is there a setting which makes the forum automatically switch to the same theme (light or dark) as the desktop (or phone)?
There is not, you have to pick one.
Is it something that can be done, or does it require an update of the discourse platform?
It seems like it’d require disabling the feature that lets users choose their color scheme, and I’m not sure that’d make people very happy.
In the discourse instance I run there are the following options for users
But I can’t find where those settings are in the admin stuff 
Yeah I see the list of themes, but no admin options like that.