I've solved my problem. But in case anyone else has this problem, here is what I was doing wrong. I forgot to map all of my paragraph styles in every chapter. This was easy to fix because I work from a template.

1. In the template's Paragraph Styles panel, I selected Edit All Export Tags from the panel menu. For each style, I mapped to the appropriate CSS style and class that I set up in my custom CSS file.

2. From the book panel, I selected all chapter files and synced them to the template's paragraph styles using the icon in the book panel's toolbar.

3. From the book panel's menu, I selected Export Book to EPUB and designated that the custom CSS file be used. (Note that ID uses that file but renames it to template.css during export.)

VOILA! A clean template.css file and the epub styled correctly.

I can't empathize enough how important it is that you remove all overrides in your files to keep your CSS clean.

I found a helpful discussion of some of these issues at https://www.creativeprogression…..he-rescue/