Add $thumbnail-border-width; refs #18150

scss/_images.scss

@@ -19,7 +19,7 @@
   padding: $thumbnail-padding;
   line-height: $line-height;
   background-color: $thumbnail-bg;
-  border: 1px solid $thumbnail-border-color;
+  border: $thumbnail-border-width solid $thumbnail-border-color;
   border-radius: $thumbnail-border-radius;
   transition: all .2s ease-in-out;
   @include box-shadow(0 1px 2px rgba(0,0,0,.075));

scss/_variables.scss

@@ -588,6 +588,7 @@ $list-group-link-heading-color: #333 !default;
 
 $thumbnail-padding:           .25rem !default;
 $thumbnail-bg:                $body-bg !default;
+$thumbnail-border-width:      $border-width !default;
 $thumbnail-border-color:      #ddd !default;
 $thumbnail-border-radius:     $border-radius !default;