Yep, ok, thanks!
So here’s what I’ve got now with some more exploration.
Seems like current default theme is stored in user options field theme_ids which is an ARRAY … and in the 1th element, so theme_ids[1].
select u.username, t.id, t.name from users u inner join user_options o on u.id = o.user_id inner join themes t on o.theme_ids[1] = t.id where theme_ids[1] <> 0
In my case it’s not even worth grouping and counting because generally users don’t seem to have selected a different default theme to the default, default one - it comes back with two users (out of a regular bunch of 300!), and one of them is … ME! 
:
I guess the issue is you can’t see what users have set on their local, by device, settings?