I googled for changing colors for the forum fonts and found a thread from 2012. I think it still may mostly be the same in 2019, but, if there's an extension for that, I'd sure want to give it a shot. The thread from the 2012: viewtopic.php?t=2151490
Anyway, if this kind of extension doesn't exist yet, I'd want to request for one.
I.E. An extension to change all the forum font colors there are.
EDIT: If I want to change all the font colors on my forum that are grey atm. and the same kind of grey ones, do I still need to change every color separately then in the colours.css?