edit background address color CSS

-- url of the page with the problem -- : www.xyznetworks.com.au/index.php/hikasho...ontrol-panel/address
-- HikaShop version -- : 4.4.3
-- Joomla version -- : 3.9.27
-- Browser(s) name and version -- : All
-- Error-message(debug-mod must be tuned on) -- : No error

I am trying to change the background color of the boxes in the image attached. I am using this css:

.hkc-md-6 {
background-color: #000000;
}

Obviously not working. I dare say I need to add the above DIV elements but this always puzzles me. Image attached re what I am trying to change. It is basically the billing and ship