It would be great if you could export the HTML & CSS that is creating the preview with one button click instead of clicking on each item individually and copy and pasting the CSS into a new CSS file. You are rendering the html already within the preview so why not allow us to export the HTML & CSS all at one shot?
If we were to do this, I'm not confident we could give you something useful. We could guess at some classes, but there would certainly be duplication of rules, and it would require a lot of refactoring before it would be useful to you. So, we're not going to try to do this.