Change border color around window (edit GTK theme)

I am using a fairly recent version of Cinnamon (5.4.12), with desktop theme Adwaita-dark (and Adwaita sometimes). When I start a gnome-terminal window, it has a very thin (1px?) white border all around it. But as I like black terminals, I configured a black background to it, but now, I cannot see the window borders anymore. This also happens with other dark background applications. It can be very confusing to know which is which when terminal windows are overlapping, or even just next to one another. I tried to change this colour with no success: - by searching in the theme files in /usr/share/themes/Adwaita, - by playing with ~/.config/gtk-3.0/gtk.css to override some configuration, - by reading other similar questions , but they are either outdated or not applying to my use case. So, **how can I change the border colour of gnome-terminal (and possibly all other) windows to white?**