I discovered the Mark all as Read button in Messages by accident - wondering how to get rid of the unread messages number (which I really dislike btw, anyone else? Why - because it’s an idea that is used extensively to make the internet more addictive than it needs to be).
I suggest putting the ticks in a seagreen circle, or rounded-corner button to make it more visible. Or writing out the actual words and putting them in a white/light grey rounded-corner button, with a darker grey stroke (like the profile drop-down top-right). I wonder if other platforms spell it out these days instead of using a symbol because people don’t see it?