public function background(): Font
    {
        if ($this->color !== null) {
            $this->turnToBackground();
        }

        $this->background = true;
        return $this;
    }
public function background(): Font
    {
        if ($this->color !== null) {
            $this->turnToBackground();
        }

        $this->background = true;
        return $this;
    }
public function background ( ) : Font { if ( $ this -> color !== null ) { $ this -> turnToBackground ( ) ; } $ this -> background = true ; return $ this ; }
function to set color as background
@return Font
function to set color as background