This file is indexed.

/usr/share/themes/Adwaita/gtk-3.0/gtk.css is in gnome-themes-standard-data 3.22.3-3ubuntu2.

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 */