Elementor\Skins_Manager::add_skin PHP Method

    public function add_skin(Widget_Base $widget, Skin_Base $skin)
    {
        $widget_name = $widget->get_name();
        if (!isset($this->_skins[$widget_name])) {
            $this->_skins[$widget_name] = [];
        }
        $this->_skins[$widget_name][$skin->get_id()] = $skin;
        return true;
    }