I did not find (nor search extensively) my old topic on this matter.
After having activated Tapatalk, I wondered if the FF Team did not reinstall or fixed old non working features.
I do not have the answer, but I realize that changing a theme colour works again, while I had problems in the past.
Is this linked to an evolution of my browser or FF, I don't know. But I thought this may be interesting for some of you.