Thank you for taking your time on the phone with me.
After looking into the issue and discussing your experience, expectations, and seeing what you were trying to do, it definitely gave me some ideas on how to improve the product and user experience in the future.
For one thing you were trying to use a simple .css page as your plugin source... Plugins are currently designed to work with complete .html pages (although pages can have any extension you would like and can contain php code)... but it expects a document with a <head> and <body> tag minimally.
I have a few comments about your usage:
1) if you only want dynamic .css pages, you can just add them all to the page, then use the show if utility that comes with framework builder so that it only shows one based on the desired trigger. Plugins and Themes don't really add any benefit to such a simple use case.
2) An implementation using themes may be a better option for you anyway... you can not only switch out .css dynamically but the html and entire page structure as well. This will allow you a much greater flexibility in skinning beyond just changing a .css file.
3) If you really wanted to use plugins, you could create a page that has the link to the css file in the head. Then use that as your plugin. Then framework builder will add the css to the page head where it is included.
I'm glad we had time to talk. Thanks again for your valuable feedback.