Firefox

Install Stylus > Write New Style > Import and then copy/paste this in. Keep in mind that I removed a lot of my specific tweaks for sites I use, because that's PII. You will encounter many more weird issues on random sites than you do with DarkReader, but if you're used to working with userCSS you'll probably have no issues fixing those. The way this essentially works is by inverting your entire browser screen, then rotating the hue so the colours of website themes aren't weird, then it inverts images back to normal. I'm sure there is a way to do this without inverting the images in the first place, but it would involve one hell of a lot more code than this. I wrote this originally in about 3 minutes.

html, iframe {
    filter: invert(1) hue-rotate(180deg);
}

img, div[background-image], div[style*="background-image"], video  {
    filter: invert(1) hue-rotate(180deg);
}

@-moz-document domain("lemmy.ml"), domain("ultimate-guitar.com"), domain("open.spotify.com"), domain("discord.com"), domain("localhost") {
/* Exemptions for sites that already have a dark mode */

html, iframe {
    filter: none;
}

img, div[background-image], div[style*="background-image"], video  {
    filter: none;
}
}

@-moz-document domain("youtube.com") {
#movie_player {
    filter: invert(1) hue-rotate(180deg);
}

video {
    filter: none;
}
}

@-moz-document url-prefix("https://www.google.com/maps") {
div[aria-label="Street View"] canvas, div[aria-label="Photo"] canvas, button[data-photo-index] {
    filter: invert(1) hue-rotate(180deg);
}

div[role="img"] {
    filter: none;
}
}