/**
  * Update all child themes relations
  *
  * @param \Magento\Framework\View\Design\ThemeInterface $themeModel
  * @return $this
  */
 public function updateChildRelations(\Magento\Framework\View\Design\ThemeInterface $themeModel)
 {
     $parentThemeId = $themeModel->getParentId();
     $this->addFieldToFilter('parent_id', ['eq' => $themeModel->getId()])->load();
     /** @var $theme \Magento\Framework\View\Design\ThemeInterface */
     foreach ($this->getItems() as $theme) {
         $theme->setParentId($parentThemeId)->save();
     }
     return $this;
 }