If needed, I have proper Steam indicator icons for Ambiant / Radiant (as well as a few others). It’d be better if they could be added to the Ubuntu Mono theme from which MATE depends, though, but I’m not sure where to offer to add them.
EDIT: I’ve just checked the Papirus theme and the icons don’t match the design of the Ubuntu Mono indicators. Mine do.
EDIT: put them there if you want to check or include them in MATE themes: https://www.dropbox.com/s/4xmb0h8up05xpb9/ubuntu-mono-panel.tar.gz?dl=0
Note: they’re in ubuntu-mono-dark and ubuntu-mono-light folders but they can be put instead in Ambiant-MATE and Radiant-MATE.