How to set background and foreground for windows and applications?

I have managed to set different background for gnome-shell-theme by editing .css.
Now I would like to change background of a window, like file manager or browser top bar (control bar).
Zorin Appearance allows to do it but not separately. Would be very nice if you would add option to set background and foreground separately.

But for now, can you please direct me to the right tag in .css?, as far as I feel it is in gtk-3.0/gtk.css file..