OK, please ignore my previous post; I now realize that if I for instance edit a Heading-4 for one chapter, that heading is only edited 'locally' for that chapter; not 'globally' for the entire book.
So, as far as I can see, I should first double-click each separate chapter to see which styles are used in it, and then edit each one.
And there's like a couple of hundreds, so now I know what to do for the rest of the day.
