Drupal - How to set the default theme programmatically

The current advice is to set this via config:

\Drupal::configFactory()
  ->getEditable('system.theme')
  ->set('default', $theme)
  ->save();

The previous method using ThemeHandlerInterface::setDefault() is now deprecated.

You might need to install it first too, I'm not sure. In case you do:

\Drupal::service('theme_installer')->install(['test_theme']);