#555 is still quite dark and should be easy to read. The real culprit is the 300 font weight - when set to 400 it's quite another matter.
I don't mean this pejoratively, but how old are you? I'm just curious as I had no problems with the color / font weight. For my sites I usually use something like #232323 instead of pure black.
I'm 25 and had to zoom to read and even then it wasn't comfortable. (not the OP)
Ah I see, upping the font-weight to 400 helped when viewing it on my laptop screen. I didn't really have that issue on mobile, though.
I’m 25. I agree with other commenters that the issue is more with the font-weight than the color; I don’t have issues reading e.g. text in #666 or even #999.