How to add constant properties to Bootstrap map

Question

Let's say I wanted to generate a responsive list of classes in bootstrap for a border: .customborder-sm-white, .customborder-md-white etc.

How would I also specify that the border for all those classes is always 3px high?

I can do the first part using the Utilities API:

$utilities: map-merge(
  $utilities,
  (
"customborder": (
    property: border-color,
    class: customborder,
    responsive: true,
    values: (
      black: #000000,
              white: #FFFFFF,
      
    )
   ),  
  )
);

But how could I add other properties to the generated map?