#17895 Added $alert-border-width that defaults to $border-width

scss/_alert.scss

@@ -5,7 +5,7 @@
 .alert {
   padding: $alert-padding;
   margin-bottom: $spacer-y;
-  border: 1px solid transparent;
+  border: $alert-border-width solid transparent;
   @include border-radius($alert-border-radius);
 
   // Improve alignment and spacing of inner content

scss/_variables.scss

@@ -525,6 +525,7 @@ $modal-sm:                    300px !default;
 $alert-padding:               15px !default;
 $alert-border-radius:         $border-radius !default;
 $alert-link-font-weight:      bold !default;
+$alert-border-width:          $border-width !default;
 
 $alert-success-bg:            $state-success-bg !default;
 $alert-success-text:          $state-success-text !default;