I think that’s right.
If you only consider Anki desktop and Ankimobile, you could probably use “reviewExtra” to set global CSS, but Ankiweb and AnkiDroid don’t seem to support "reviewExtra"
. I think it’s better to add the CSS to each note type.
It might be technically possible, but I think it would require complex javascript. The background-color
property for <img>
tag has no effect unless the images have a transparent background layer, so I don’t think it’s necessary to restrict the target images.