It would be good to have a blacklist, to disable it on some sites.
Maybe add an option so that only pure gray text is changed, so that coloured text is not altered.
There is also the dilemma of dark backgrounds. Ideally lighter than 50% text should be lightened, but how do you know what the background is?
What I like about the add-on is it keeps things fairly simple.
I see it needs a restart to take effect, but this is not made clear, so people may think it doesn't work.