getThemeName example

$placement = FALSE;
    if ($this->request->query->has('block-placement')) {
      $placement = $this->request->query->get('block-placement');
      $form['#attached']['drupalSettings']['blockPlacement'] = $placement;
      // Remove the block placement from the current request so that it is not       // passed on to any redirect destinations.       $this->request->query->remove('block-placement');
    }

    // Loop over each region and build blocks.     $regions = $this->systemRegionList($this->getThemeName(), REGIONS_VISIBLE);
    foreach ($regions as $region => $title) {
      $form['#tabledrag'][] = [
        'action' => 'match',
        'relationship' => 'sibling',
        'group' => 'block-region-select',
        'subgroup' => 'block-region-' . $region,
        'hidden' => FALSE,
      ];
      $form['#tabledrag'][] = [
        'action' => 'order',
        'relationship' => 'sibling',