I made it so that when I encounter an image with colors that make it hard to read, I could easily invert it. I didn't bother trying to save that permanently, since 1) I only read an article once, so it's not like I'm inverting the same element every day; 2) cases where I see the same element repeatedly and it ought to be inverted are already handled by Dark Reader; and 3) persistence is a much harder engineering problem to solve.

Having made this for my personal use, I decided to share it publicly and repay the generosity of others who made their software freely available for my use. I never claimed it would be useful for everyone, I just described the add-on as it is. It's a context menu item that inverts the element you clicked.

If you have some kind of persistent contrast problem with a site, you can always take it up with the site developers or the kind folks who develop Dark Reader, which maintains a permanent list of inversions.