$u-border-widths: (
  s: 1px,
  m: 3px,
  l: 4px
) !default;

// @each $key, $border-width in $u-border-widths {
//   .u-border-width-#{$key} {
//     border-width: $border-width;
//   }
// }

@mixin get-border-width($size_key){
  border-width: map-get($map: $u-border-widths, $key: $size_key );
}