postinst is in gnome-themes-standard-data 3.10.0-1ubuntu2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | #! /bin/sh
set -e
update-alternatives --install /usr/share/icons/default/index.theme \
x-cursor-theme /usr/share/icons/Adwaita/cursor.theme 90
|