How can i change the width of a field in sale order line of a sale order when it is appearing on sale order form view?? i tried to change the field "Product" using css codes

openerp .oe_list_content thead  tr th:nth-child(2) 
{
width: 225px;
}

and

[data-id="product_id"]{width: 20px;}

But it effects all "Product"(product_id) fields of OpenERP and also the tree view. Please help.