Another option can be: after log in open a form with a html snippet that redirects to a specific index page that includes specific css (example: index-mycustomer.html) just like index-dark.html etc.
When a user logs in a class is added to the body of the index html. So if an administrator logs in the class "role-administrator" will be added to the body. You can then use this class to style the application differently per user role.
There are already some forum post about that subject. See for instance this post: make parts of your theme configurable Try searching on dynamic or theme.