Change style/pngs of checkout address

-- HikaShop version -- : 2.6.0
-- Joomla version -- : 3.4.5
-- PHP version -- : 5.5.30

Hi

In the address section of the checkoutproces the little png buttons edit, cancel, ok and delete are very small so customers miss them and click on the checkout next button in stead, which results in an error and confusion.

So, I want to change the buttons.

but, I can't find this section in the custom css. Is there a way to change the styling of the icons in the configuration?
I can ofcourse replace the hikashop images in the /media/com_hikashop/images folder but then they will be replaced by the next update of hikashop.