You should just convert to .svg the white icon, the rest of the button will look better in CSS.

That may be the case, but there's many contexts where you'd use this button where you can't alter the page's CSS. Perhaps the prime example being GitHub READMEs.

I'm not on a retina display but that doesn't look as nice. The letters are so thin they start to disappear even before using the light-purple-on-dark-purple font.

the font does look thinner but I like it (doesn't look any more invisible from my eyes)

The fact that on a Mac the font is thinner makes me wonder if you can even see it on Windows?

Judging by my windows machine and using just my eyes, yes, it's a little thinner, and no, there's no problem reading it. The faded-grey-ness of the words "to Heroku" is a much bigger legibility problem than the minor change in font weight.

Very impressive