I can't believe this isn't being shouted from the rooftops. Same problem has been plaguing me with the reader view on Firefox for iOS for ages now. Even if it were at least an option I'd be grateful, but a medium grey, especially on OLEDs, is not cutting it.
Please add an option for black theme in mobile Firefox! Although the purple ish grey is fine for most I know many of us would love a pure black AMOLED theme for our phone)
Dark mode with all websites in dark mode for Firefox Android
I would like that in Firefox Android when you put the dark mode all the pages (including the google pages) would be put in dark mode without having to install an extension that does it.
Or if you don't want to put it in black, modify it to your preference for example: