Add $dropdown-border-width; refs #18150

scss/_dropdown.scss

@@ -51,7 +51,7 @@
   list-style: none;
   background-color: $dropdown-bg;
   background-clip: padding-box;
-  border: 1px solid $dropdown-border-color;
+  border: $dropdown-border-width solid $dropdown-border-color;
   @include border-radius($border-radius);
   @include box-shadow(0 6px 12px rgba(0,0,0,.175));
 }

scss/_variables.scss

@@ -306,6 +306,7 @@ $form-icon-danger: "data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My
 
 $dropdown-bg:                    #fff !default;
 $dropdown-border-color:          rgba(0,0,0,.15) !default;
+$dropdown-border-width:          $border-width !default;
 $dropdown-divider-bg:            #e5e5e5 !default;
 
 $dropdown-link-color:            $gray-dark !default;