_sass: Full-width cards on smaller devices

_sass/custom/mixins.scss

 @mixin abstract-card() {
-  border-radius: $border-radius;
   box-shadow: $box-shadow;
+  margin-left: -$gutter-spacing-sm;
+  margin-right: -$gutter-spacing-sm;
   margin-bottom: $sp-4;
 
+  @include mq(md) {
+    border-radius: $border-radius;
+    margin-left: 0;
+    margin-right: 0;
+  }
+
   >*:last-child {
     margin-bottom: 0;
   }