This file is indexed.

postinst is in gnome-shell-common 3.28.1-0ubuntu2.

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.

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#!/bin/sh

set -e

if [ "$1" = configure ]; then
  update-alternatives --install /usr/share/gnome-shell/theme/gdm3.css \
    gdm3.css /usr/share/gnome-shell/theme/ubuntu.css 10
fi