Oh my god this would actually be amazing! Needs some thinking about though. Like if you want them all to be the same colour but not the default one, having to go in and set it for each of your accounts would be annoying. So maybe the colour setting stays where it is but then each account has kind of an optional "override"? This is just off the top of my head I'm sure someone better at UX than me has a better idea.

I love this idea. I just the other day subbed a bunch of communities on the wrong account because I forgot I switched.

I only have one account but I still think this would be a great idea. One default colour theme, but have the option to override it for each account.