Fixes #12914: Darken immediate children hr elements in jumbotrons

less/jumbotron.less

@@ -19,6 +19,10 @@
     font-weight: 200;
   }
 
+  > hr {
+    border-top-color: darken(@jumbotron-bg, 10%);
+  }
+
   .container & {
     border-radius: @border-radius-large; // Only round corners at higher resolutions if contained in a container
   }