/usr/share/themes/HighContrast/gtk-3.0/gtk.css is in gnome-accessibility-themes 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 | /* HighCOntrast is part of Gtk 3.14, this file is no longer used */
|