Really ... can it be this easy?
I was poking around again ...
Calibre seems to rewrite everything. But in MOST epubs, there is ONE definition which applies to the vast amount of body in the book. Here's an example:
It's easy to see which is the main body since Calibre appears to re-assign the attribute on every single paragraph.
Here is ONE simple change that turns a left justified fixed very small book into a justified, user-controlled variable size font (the Kobo font button works) e-book:
This is the beauty of style-sheets: they are universal throughout the document(s).
Instead of "trashing" stylesheet.css ... make one small edit to the section which controls the main body of the book. In this example, it was ".calibre3" and that was easy to find since every paragraph in the html is labelled that way.
"Normal" appear to be a default which allows the e-reader to take control.