Front.id

When you enable a Drupal module or profile, it may be useful to activate and set a theme for the end user and/or Drupal administration.

For this type of situations I leave you a snippet of code that is placed in the hook_install() of some module of your own that you have out there:

function foo_install() {
  $theme_list = [
    'stark',
    'adminimal_theme',
  ];

  // Install themes.
  \Drupal::service('theme_installer')->install($theme_list);

  // Get theme manager
  $system_theme = \Drupal::configFactory()->getEditable('system.theme');

  // Set default and admin themes.
  $system_theme
    ->set('default', 'stark')
    ->set('admin', 'adminimal_theme')
    ->save();
}

And that's it. When you activate the module "stark" will be activated for the end user and "adminimal_theme" for the admin part.

NOTE: This code logically works also in the hook_update_N() and hook_uninstall().

Bye!