/usr/share/themes/Adwaita-dark/gtk-3.0/gtk.css is in gnome-themes-extra-data 3.28-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 | @import url("resource:///org/gtk/libgtk/theme/Adwaita/gtk-contained-dark.css");
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