enforce $card-border-width on cards

src/scss/_variables.scss

@@ -302,6 +302,7 @@ $btn-font-weight: $font-weight-medium !default;
 $btn-border-radius: 3px !default;
 
 //cards
+$card-border-width: $border-width !default;
 $card-border-color: $border-color-transparent !default;
 $card-border-radius: $border-radius !default;
 

src/scss/ui/_cards.scss

 .card {
   box-shadow: $card-shadow;
+  border: $card-border-width solid $card-border-color;
 
   @media print {
     border: none;