Haven’t tried Flu.x, but it’s a good idea: I use DarkReader. I also like that it can replace the fonts selectively. I find flare-tip fonts are easy to read like sans-serif, but better for white-on-black than. They’re not more commonly used, because they require very high screen resolution, and just as ultra-high-res screens became the norm on computers, people started using handheld devices. (Right now, I’m set to quicksand font, which isn’t flared, but has very rounded letters.)
The only problem is that DarkReader occasionally makes pictures into negatives.
(Dark reader doesn’t filter out blues, it simply changes dark-on-light writing to light-on-dark, generating about 90% less light.)