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();
    }

}