TL; DR. Is that because of the color fringing associated with subpixel antialiasing (which is the default)? Most people don’t notice it, some people hate it, and you seem to be in some middle ground.
If I inspect my Atom (0.95.0), I see
-webkit-font-smoothing: auto on text. Does that match what you see?
I would verify that
-webkit-font-smoothing: subpixel-antialiased reproduces the problematic output you saw before. I can’t really check myself, since I’m not sensitive enough to notice the effects on color.
If that’s the case, searching webkit-font-smoothing will give you lots to read, and tons of debates over what’s better.
About font width: it appears that OS X makes the font bolder when using subpixel antialiasing. But most people fiddling with this seem to dislike it, because that’s not how those fonts are designed to look. I’d just look at how to increase the font weight or pick another font — people seem to use all sort of hacks for that (I’m no web designer though, I can’t really judge).