@Nick, Referencing a css filename in the Extra CSS box should work, it certainly did at one time as I used to do it all the time.
Perhaps take a step-by-step approach.
1. Try Agama's suggestion but on a single book only.
2. If that works, replace the css with the css filename (as you tried originally) but still only converting a single book.
3. If that works try step 2 but highlight 2 books for conversion.
At what point does the extra css no longer work?
P.S. Are you sure the word
none is valid in this line of CSS?
Code:
body, div, p, h1, h2, h3, h4, none { margin: 0; padding: 0; }