Here is a step-by-step way that eliminates most of the formatting/font issues that your css isn't working on in epubs:
1. In calibre, select the book that is giving you problems
2. Right-click that book, and choose Tweak epub from the contextual menu that appears
3. A folder will open up with a bunch of files
4. Delete any file with ".css" in that folder or the sub-folders
5. Delete any file that ends with ".xpgt" - it is usually called "template.xpgt" but occasionally comes in different names
6. Open the ".opf" file using a text editor and delete any lines that have the ".xpgt" file written in them
7. Go back to calibre and choose the button that says rebuild epub
8. Convert the file again so that your css or the calibre default css is written into the epub.
9. Delete the epub off of your reader and add the new epub to the reader.