[JESimmons]

I'm trying to put a border around an image on a page. I open the book in calibre, go to the page, highlight the image, right-click and go to inspect. In the inspection screen, I go to the css in the right column and enter margin: 10px.
I exit from the inspection area and look at the calibre reader page and the image is bordered exactly as I want. I close the reader.

I reopen the reader and all the changes are gone - the image again has no border and in the inspection area, the added css is gone.

I hope this isn't the stupidist question in the history of the forum, but how do I save the changes in css that I make in calibre?
Thanks in advance,
John