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.
Definitely been there for more than a month already. I’d say at least six months, but probably more.
Unfortunately, Firefox For Android never was terribly widely used and they had to do a major rewrite twice already, so yeah, things may not be documented quite as thoroughly.