The newest version of Firefox has a little icon resembling an open book at the right side of address field that does something similar. Quite handy :)

Nice. I'd prefer that was the _default_ browser view, frankly.

I recommend the "zap colors" bookmarklet from https://www.squarefree.com/bookmarklets/zap.html

It simply changes all colors so that text is black-on-white, but leaves everything else unchanged.

Sadly, it's not just colours that are the problem.

Font sizes, poorly sized and laid-out pages, overlapping divs and other elements, animiations....

Readability fixes most of that quickly.