One thing we should do here for sure is give different CSS classes to various warnings.
At the moment it has .composer-popup-container .custom-body we should be able to target this better with say .composer-popup-container .custom-body.group-mention
Slack have these options:
I think what @mrjn is proposing here is even higher fidelity.
Perhaps our first move here is to add better CSS classes. EG: for this case .group-mention.group-member