Hey, author here — sorry about that. I don't use/didn't test with Firefox, but it looks like it's not setting the page background properly in dark mode. I'll get that fixed!
Consider also testing "-webkit-font-smoothing: antialiased;" on non-retina screens. When removed text became somewhat thicker and significantly more readable.
I'll try that tonight! Any suggestions for testing/seeing how the anti-aliasing works when I don't physically have non-retina screens?
Done — I think antialiasing looks better on Retina screens in dark mode so I kept it only for that case (removed it for non-dark mode or non-Retina).