Thanks - this helps. I've been using an external monitor, and, while the side bar text (Finder, Mail), is showing up as grey with little contrast on the external monitor, if I use my laptop's built-in retina display I can see that the side bar text is indeed black (or a very dark grey?). It's much more legible on the built-in retina display, although it would be better if the side bar font was adjustable to make it bolder / thicker / more black.
I've experimented with all of the accessibility settings, as well as the general settings. I prefer the Light appearance over Dark, but agree that Dark has much better contrast.
I'm currently experimenting with the different display color profiles on my external monitor, and am trending in the direction of something I can read more easily (although photos look awful now). I've upgraded operating systems so many times in the past and have never had this issue before.