Yeah, it's probably about font size.
The screenshots were taken on Ubuntu + Chrome. I came home and checked on my Mac, and it seems like Fira isn't always that bad. Ubuntu just stretches it for some reason and it looks horrible, like in my "Before" screenshot. As for Concourse, I managed to make it look like my "After" screenshot on both platforms by setting -webkit-font-smoothing: antialiased. But that seems brittle. So yeah, fonts are tricky and I withdraw my proposal, at least until I get ten different machines to test it :-)
I've now tested the grey theme on several different Linux machines (running debian, Mint, and KDE), in Chrome and Firefox, and am unable to reproduce that particular bug (I don't have access to an Ubuntu box at the moment, I'm afraid).
I'm given to understand that Linux has certain font rendering settings which may be adjusted—is that something you're able to try doing? (If not, I totally understand.)
Might as well go with a bang.