Index: wp-content/themes/twentyfifteen/css/blocks.css
===================================================================
--- wp-content/themes/twentyfifteen/css/blocks.css	(revision 56614)
+++ wp-content/themes/twentyfifteen/css/blocks.css	(working copy)
@@ -473,7 +473,6 @@
 /* Seperators */
 
 .wp-block-separator {
-	border: 0;
 	max-width: 100px;
 }