I'm using a userChrome.css file that works very well with Firefox 158 on Arch Linux. I'm looking for a way to make dark icons stand out better, since I'm also using a dark theme. It seems to me that the following filter might be a good solution:

filter: drop-shadow(1px 0 0 rgba(255, 255, 255, 0.7)) drop-shadow(-1px 0 0 rgba(255, 255, 255, 0.7)) drop-shadow(0 1px 0 rgba(255, 255, 255, 0.7)) drop-shadow(0 -1px 0 rgba(255, 255, 255, 0.7)) !important;

The problem is that, for one thing, you have to target all the places where favicons appear:

• the tab favicons;
• the Bookmarks Toolbar, as well as its folders and subfolders;
• the "Bookmarks" menu in the menu bar;
• the favicons in the bookmarks within the hamburger menu;
• the bookmarks in the sidebar;
• the bookmarks in the library;
• am I missing any?

On the other hand, this filter must not apply to Firefox’s internal icons (for example, the folder icon, the default globe-shaped favicon, favicons for error pages such as "Server not found" or 404 errors, etc., and favicons for about: pages, etc., etc.).

How can I do this? Thank you very much in advance for any help you can provide!