In the middle of the article, there is a brief discussion about icons and colors.
I also think that well-coordinated color icons are good for readability and usability.
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.
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.