Put a lighter background behind some topic icons on dark themes
Favicons often have a transparent background, and are designed for the normal background they're displayed on - the light gray background that browser tabs use by default. Because of this, some dark icons are almost invisible when you're using the darker themes:
We should have a way of putting a lighter background behind some of them to fix this for the dark themes. I don't think we want a lighter background behind all icons, because most of them look totally fine.
We could just manually define which ones need to have a background shown, but overall it would probably be ideal to try to determine this automatically inside the script that generates the topic icon CSS. I'm not sure how difficult that would be, but it would probably need to (a) determine that the icon has a transparent background and (b) check if the non-transparent pixels seem to be dark overall.