[retiredbiker]

I'd look at it through the editor. I can't imagine what converting could do unless you know what is inside them in the first place. The coding possibilities are infinite. Just stuffing some css into a conversion is not going to fix all the coding that may be there.

First, Open them in the editor and run Tools-->Remove unused css in both books; that will get rid of any clutter.

Then look at both books and see just what the difference, if any, is between the css files. If they are identical, it answers your question. If not, what do you want to do? Choose one, and make the other book match?

Next question--is this a reasonable task? Is the css simple enough to be fairly easily understood? If you have two books with thousands of lines of css and it is all different...well, I'd probably quit right there. If it looks reasonable, then to continue:

Export the css file you like best from one book and import it into the other. Then select all the text files and right-click, then link in the new css file. Then you have the fun of using search and replace to make the text selectors in the book match those in the new css file. Depending on the coding, this could be anywhere from a trivial task to a horrible one; can't say without seeing the books. In the end you should be able to delete the original css file without causing any errors or leaving anything behind.

I've done this a few times, if the coding was pretty similar in the two books. If it turns out to be very different, or very complex, consider blowing it all away and putting your own (simple) css into both books.

If you ever use the EpubMerge plugin, you often face this, only within the one merged book you have made. You might make an anthology of short stories, and you want them to all display the same way. So you might have, say, 10 or more css codings to harmonise. Fun and games!

Now here is a late thought. If all you want is to be able to copy a page from one book to the other (for which you need the editor anyway, and the concept of a "page" here is very fuzzy), and have it "work" -- all you really have to do is export the css files and import each into the other book, if that makes sense. Then link them to all text files. Then any page from either book will find something to match to in the other book. Downside--if the same selector names are used with different coding in the different books, the results could be interesting.