I would not characterize the [dead] comments (which are #ddd) as "a typical comment". They're designed to be hard to read. If I turn off showdead (which I believe is an opt-in setting), it looks like the lightest comments are #9c9c9c, which is a 2.5 ratio (and of course are also comments which are intended to be less prominent).

They ought to have a button to put on a high-contrast stylesheet, in fact I bet some hacker has already done it.

I'd assumed anyone with visual impairment—but not outright blindness requiring a screen-reader—was using custom stylesheet stuff pretty much constantly all over the web. Controlling the appearance of sites client-site used to be a basic browser feature. I know that's receded into the background, but is something similar (either some accessibility feature, or a plugin) not the norm for those who need them? If not, that sucks, because control used to be so firmly on the client-side for those things and that was a good thing.