Add a mixin for borders

Author: grahamc

src/sass/_mixins.scss

@@ -28,6 +28,17 @@
   }
 }
 
+@mixin border($size) {
+  border-width: 1px;
+  border-style: solid;
+  border-radius: map.get(tokens.$border-radius, $size);
+
+  border-color: map.get(tokens.$border-color, dark);
+  @include light-mode {
+    border-color: map.get(tokens.$border-color, light);
+  }
+}
+
 @mixin transition($property, $duration) {
   transition-property: $property;
   transition-duration: map.get(tokens.$duration, $duration);