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Yes! I've just managed to change the PDF viewer in #Firefox to have light grey text on a dark grey background, rather than the white background that hurts my poor eyes. (The UI is already dark mode, thanks to ShadowFox.)

In ~/.mozilla/firefox/[profile name]/chrome/userContent.css:

@-moz-document regexp(".*\\.(p|P)(d|D)(f|F).*") {
#viewer.pdfViewer > .page {
filter: invert() hue-rotate(180deg) contrast(0.8);
}
}