This assumes your app already has some views for the primary css-framework you target, and now you want to support more frameworks.

This checklist assumes that your initial templates support Bootstrap4 and you want to also support Bootstrap5 and that your templates are either in the root folder or distributed in subfolders of the root folder. 

1. Create a new folder called bs5 for the new second css framework you want to target.
   You can also target any other framework. Here is the recommended list of names.
2. Copy all your template files into that folder
3. Activate CSS-Framework Polymorphism
4. Test to verify switching works

You can now make your changes to the copied templates as needed.