I have a bookmarklet, which I've shared here: http://codepen.io/anon/pen/wKpewj (there are lots if you search bookmarklet + {something}). It makes the text 1em, not bold/thin, black on tan.

Something a bit better could preserve headings, bold words etc.

Firefox's Reader Mode sometimes works, although not on this site.

Yes, you can use plug-ins like Stylish. It works well for tweaking sites you read regularly, HN say, though personally I find it too much messing around for a site I'm just visiting quickly. I'm more likely to just open the developer tools in whichever browser I'm using and look for rules that set a tiny font-size, a font-weight of 300, a faded color, etc. Often just disabling a couple of those will make a page dramatically easier to read, so I do recommend spending five or ten minutes figuring out how it works in your preferred browser. Even then, I sometimes have to admit defeat. I hate the idea of criticising style rather than substance, but with a site as awful as this one you can spend several minutes trying to make it comfortable to read and still fail.