I modified reader.color_scheme at about:config and it worked flawlessly on both my home and work PCs, but not on the phone, it just stays white.
I’m using Fennec F-Droid btw, which is basically a fork that just removes the tiny proprietary bits from the browser, and is manteined by the FF devs themselves as far as I’m concerned.
What did you change it to?
just “dark”.