Can I set the theme for certain groups? For example: registered users get theme X, admins & mods get theme Y. Something like the 'Force on users' option but for specific users.
for each theme at its
properties you can select which group can use that theme.
this feature can be used for your purpose. i.e. for each theme select a single group or a couple of groups ..
a brief of editing theme properties (for MyBB 1.6.x)