Menu highlight background color

[Stanzer]

Hi, I've been customizing my monterail (full-dark) and I can't seem to find how to change the highlight background colour of menus. It stays light grey when I would like it to be orange (the gnome canta-theme color scheme).

Any help on where I should amend the css file would be greatly appreciated.

Thanks!

(Upper part of image: what I have, lower part, what I would like to achieve)

[Sadi58]

I've been searching for this for a long time as well. It seems this element is locked in Mozilla code, and nobody knows (or cares) how to unlock it. ;-)