Index: wp-includes/css/dist/block-library/style.css
===================================================================
--- wp-includes/css/dist/block-library/style.css	(revision 53178)
+++ wp-includes/css/dist/block-library/style.css	(working copy)
@@ -177,6 +177,7 @@
 .is-style-outline > .wp-block-button__link:not(.has-background),
 .wp-block-button__link.is-style-outline:not(.has-background) {
   background-color: transparent;
+  border: 2px solid;
 }
 
 .wp-block-buttons {