https://github.com/black7375/Firefox-UI-Fix/wiki/%5BArticle%...
However, it is difficult to apply it universally to support a variety of colors. If it is similar to the background color, it is difficult to distinguish and there may be contrast issues depending on the light/dark theme. I think it's just the ease of development of a solid color icon that matches the color of the text.
I just wish it was something more 3D and "skeuomorphic" instead of just making a flat gray arrow into a flat green arrow. For example, if Chrome used a yellow star instead of a white star outline, I bet a lot of people would say it looks ugly as hell and sticks out like a sore thumb, but I'd prefer it. I can barely tell these monochrome icons apart.
Space isn't the only issue. Fewer options generally (very generally) yields better design - it's easier to find things, less distraction, cleaner, etc.
You seem to be knowledgeable about UI/UX. May I ask you a question? I have a theory that monochrome icons are worse than colored icons. Do you know if there are studies about this or if there's any consensus? Thanks in advance.