Setting up a dark theme made me inquire into where this grey background colour comes from:
I do not have any black in my colour scheme so I suppose that the grey is calculated as a lighter version of the complementary colour of the primary colour (white, in my case).
While I can see the reasoning behind it, it doesn't really work if your black theme is using anything outside the black/grey spectrum as secondary colour. Perhaps it's a matter of taste, but I'd probably prefer that navigation button background colour (or whatever it's called) to be based on the secondary colour.
I am not saying that every colour should be chosen manually (discourse is generally doing a great job in calculating colours - mainly shades and tints, I believe - based on the few customizable ones) but this particular formula seems a bit too constraining.
Perhaps it can be solved with a single switch that allows the user to switch the color base between primary and secondary colour?
[x] Use secondary colour as base for button backgrounds