/**
 * Implements hook_form_system_theme_settings_alter().
 */
function {{ machine_name }}_form_system_theme_settings_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state) {
  // Add a checkbox to toggle the breadcrumb trail.
  $form['toggle_breadcrumb'] = [
    '#type' => 'checkbox',
    '#title' => t('Display the breadcrumb'),
    '#default_value' => theme_get_setting('features.breadcrumb'),
    '#description'   => t('Show a trail of links from the homepage to the current page.'),
  ];
}
