Color contrast enhancer add-in for browsers?

I had some eye surgery recently and lost about 20% of my ability to detect contrast. Is there a browser add-in that would let the end-user specify, on a site-by-site basis, a contrast percentage increase, while keeping the site’s CSS colors otherwise in the same palette? I don’t need a full-blown high-contrast mode, just the ability to darken up some light text where there’s not much contrast between the text color and the background color.