/usr/share/themes/Adwaita/gtk-3.0/gtk.css is in gnome-themes-standard-data 3.18.0-2ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 | /* Adwaita is the default theme of GTK+ 3, this file is not used */
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