Hi,
It comes from the line:
$html= '<a rel="nofollow" class="hikashop_cart_button'.$classname.'" '.$options.' '.$url.$ajax.'>'.$name.'</a>';
of the file administrator/components/com_hikashop/helpers/cart.php
But I would recommend instead to edit the file "quantity" via the menu Display>Views and change the lines:
echo $this->cart->displayButton(JText::_('ADD_TO_CART'), 'add', $this->params,$url, $this->ajax, '', $max, $min);
to:
echo str_replace('hikashop_cart_button','button-default',$this->cart->displayButton(JText::_('ADD_TO_CART'), 'add', $this->params,$url, $this->ajax, '', $max, $min));
That way, the change will be as a view override and you won't loose it when you update HikaShop in the future.