Neat idea – I did a little digging and found…
- You probably can’t assign a default theme per group:
2018: How to assign a theme to a group? – Sam said: “There is no way to do this, it sounds like an interesting request especially when extended to other parts like ‘default watched categories’ for a group and other user defaults. You would need a plugin for it now.”
- There’s a way you could control what themes are available for groups:
2019: Show/hide themes for specific groups? – awesomerobot came up with a CSS method to manipulate the themes available to a user based on primary group. The CSS Classes for Current User’s Groups component already mentioned could be used to expand this to any group.
- This is a bit of a tangent, but could be part of an alternate approach:
2022: craigconstantine developed the Umbrella-CAST component to selectively display different content in the same topic depending on the user’s group.