As far as I can tell, you need to look for "placessidebar" in: /usr/share/themes/Ambiance/gtk-3.20/gtk-widgets.css
But there's a lot of complexity there. I can't tell what does what.
As far as I can tell, you need to look for "placessidebar" in: themes/ Ambiance/ gtk-3.20/ gtk-widgets. css
/usr/share/
But there's a lot of complexity there. I can't tell what does what.