public function addCustomBodyAttr(string $attr, string $element, bool $first = false) : Bootstrap4Card
    {
        if (in_array($element, $this->body_elements) === true) {
            $this->body_attr[$element][] = $attr;

            if ($first === true) {
                $this->body_attr_first[$element][$attr] = true;
            }
        }

        return $this;
    }