I have a legacy gtk.css that was working fine on Ubuntu Xenial, I've just upgraded to Bionic (2018) and the same code now is not working anymore:
$ cat .config/gtk-3.0/gtk.css
this code was needed to remove an ugly black border from windows as explained here Remove black margin from elementaryOS applications when using Awesome Window Manager.
I tried the selector
.frame>border in the debugger
$ GTK_DEBUG=interactive io.elementary.calculator but to no avail.
Anyone that follows GTK devel and knows how to update the CSS code? I tried to look into the official docs but couldn't find anything related to .window-frame having changed.