express-8.x-1.x-dev/demo_express/demo_express.install

<?php
/**
 * @file
 * Install, update and uninstall functions for the profilename install profile.
 */

/**
 * Implements hook_install().
 *
 * Perform actions to set up the site for this profile.
 *
 * @see system_install()
 */
function demo_express_install() {
  // Can add code in here to make nodes, terms, etc.
  \Drupal::configFactory()
    ->getEditable('system.theme')
    ->set('default', 'demo_theme')
    ->save();
}