Hi Robert,
One other way is defining a dynamic URL for your external style sheet (eSpace properties), say '/MyApp/DynamicCSS.aspx'
Then you just have to create an entrypoint/screen with that same name and return the proper web.css through the Download widget in the Preparation action of that screen.
You can then add further logic to the Preparation such as identifying the user and provide her the proper stylesheet.
Let me know if it helped.
Kind regards,
Pedro