I tweaked it a tiny bit since overscroll on FF on osx was showing as white, figured I'd share it back. It's not perfect, but the BG color differences are close enough to not bother me.
! Invert the colours of the site, making a dark mode that I like
! `invert(95%)` does most of the work,
! but you can tweak the hue-rotate value to fine tune the colours
news.ycombinator.com##body:style(filter:invert(95%) hue-rotate(200deg); background: white)
news.ycombinator.com##html:style(background: black)`
For instance, I use it to switch HackerNews to dark mode, and restrict the width of comments to about 10-12 words: https://gist.github.com/aclarknexient/c39c83f2f97c3c6b1c307c...
It's a great alternative to GreaseMonkey or other user style extensions.