Changing Text And Background Colour In iBooks

Wonder if you can help with iBook problem

I'm trying to change the text to white and the background to black. It should be simple enough to do: open book, find the two A. A, where one is bigger than the other but I can't find the A. A !

While most of my catalogue are free downloads PDF's and the like I re-downloaded a book I wiped over a year ago and I was able to change that text and backgrounds but I can't figure the rest !

Is the problem because these were downloads from the web and not coming direct from Apple's store? I kind of think it is.

I would like to reset ALL the books, instruction manuals and so on. Can I do it? My system software is up to date and I'm running

Many thanks for any help you can offer. I operate an iPad mini and iPhone 6s

Kevin