The \Magento\Theme\Api\Data\DesignConfigDataInterface is implemented by the \Magento\Theme\Model\Data\Design\Config\Data class

[dmitrii_fediuk]

magento/magento2/blob/958164/app/code/Magento/Theme/Api/Data/DesignConfigDataInterface.php#L14-L14

magento/magento2/blob/958164/app/code/Magento/Theme/etc/di.xml#L18-L18

magento/magento2/blob/958164/app/code/Magento/Theme/Model/Data/Design/Config/Data.php#L12-L12