Adding CSS class to custom fields

Hi,

That shouldn't be necessary. You can target specific custom fields without having to do that.
For example, if you want to target only the input of the quantity custom field on your page, you could use that CSS:
#hikashop_item_custom_quantity input{
}

But then, you can always override the view file where the fields you want to target are being displayed, to add custom CSS classes. But it's more complex as it will require to add some simple custom PHP code.