class ThemeAdminForm

class ThemeAdminForm extends ConfigFormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'system_themes_admin_form';
  }
  
  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'system.theme',
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, array $theme_options = NULL) {
    // Administration theme settings.
    $form['admin_theme'] = [
      '#type' => 'details',
      '#title' => $this->t('Administration theme'),
      '#open' => TRUE,
    ];
    $form['admin_theme']['admin_theme'] = [
      '#type' => 'select',
      '#options' => [
        '' => $this->t('Default theme'),
      ] + $theme_options,
      '#title' => $this->t('Administration theme'),
      '#description' => $this->t('Choose "Default theme" to always use the same theme as the rest of the site.'),
      '#default_value' => $this->config('system.theme')
        ->get('admin'),
    ];
    $form['admin_theme']['actions'] = [
      '#type' => 'actions',
    ];
    $form['admin_theme']['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Save configuration'),
      '#button_type' => 'primary',
    ];
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    parent::submitForm($form, $form_state);
    $this->config('system.theme')
      ->set('admin', $form_state->getValue('admin_theme'))
      ->save();
  }

}