There's a few high contrast plugins for browsers. They don't work well.
What I've found is that for the most part, resetting the fonts and color to black works. There's a few wacky sites with bizarre colored backgrounds, but those are rare. It's mostly super light grey on almost-but not quite so light grey with some font where 50% of the characters are subpixeled to a point of not being rendered.
So let's make the text black and the font times.
Plugins are way too hard. Really what there needs to be is a button that runs this. For now, just copy and paste this into your console when you need it:
Ah, it makes life so much better. I can read shit.
So the new idea I have is to have something like greasemonkey filters and some global accessible css page. So the idea is that there'd be a drop down to create your own CSS overrides for some crappy site and then you can save it in a wiki like fashion so that other users can also experience a non-bullshit user-experience where they can decipher the content on the screen.