I could tell you the story, but mokotower at least is not gonna like it 
ok, ok....
Once upon a time there was a request to make settings persistent independent of cookies.
And other requests of a similar nature, to make them associated with the account.
Then, along came the time to implement the “Player Notes” feature, which triggered an improvement of the way that all our preferences are stored and synced.
It was very natural at that time to simply make all preferences synced for the account across all device by default, so that’s what happened.
The thinking was, I think, that if there were settings that were device specific, we’d hear about it and do something at that time.
That was quite a while ago … I don’t think we heard much.
I can see that theme setting might be a nice one to have device-specific. Food for thought…