Even though this is said to be for dark text on light background, it also seems to work pretty well when using some addons for black/dark background with light text. Although it does have some problems/quirks.

When using the extensions "Night Mode Pro" or "Owl" (interchangeably on various websites, sometimes italicized/underlined text will turn black on the dark background.
For an example - when using "Night Mode Pro" on the following website: http://forum.luminous-landscape.com/index.php?topic=121095.new#new

It would also be nice if the toolbar button acted as a permanent on/off switch, and/or was settable according to website domain, subdomain, or url. It can get annoying to always have to switch it on when going to a new website - or even the same website in a new/different tab.

Still very helpful though, when needed.. Even for my dark/black backgrounds.

Letting the punishment fit the crime department: The idiots who have been pushing low contrast type should be put in a prison with ones who write novels by pasting letters cut from magazines onto pages (think ransom notes), the ones who stopped using words on signs in favor of hieroglyphs, the ones who make the power switch almost impossible to find on electrical appliances, and doctors who write important medical information in an indecipherable scrawl, and they should all be forced to use each other's annoying work. Review by vdicarlo: Font Contrast Fix has unfortunately become an essential extension. It does the job, and I am grateful to Fushko for giving it to us. Suggestions: global default and settings by site and page, apply fix to text boxes like this one that are currently unaffected while you are working in them.

3/22/2018 - I do not perceive any effect when used on my Opensuse Linux Leap 42.3 XFCE box with Firefox 64 bit version 52.7.2 Seems like this happened some time ago. I have tried uninstalling and reinstalling. Perhaps you could post or send me the url of a site where its effect should be very noticable. I would be willing to try to troubleshoot. This extension was too useful to lose. My domain is dicarlolaw top level com where I get mail at my username above.

This is super clean and useful, except when I am on a page that uses gray text on black background, in which case it makes everything black on black. It would be really awesome if there was the ability to select a custom color if I wanted to.

I think it's better to make 2 modes:
1. Remember the position of the button for each of the site visited.
2. Don't remember (mode as work now).