Twenty Fifteen: Ensure border-bottom styles are applied to the Button Block.

This issue was related to the `:where()` selector, which also affected Twenty Twenty (see #55824).

Props umesh84, sabernhardt.
Fixes #55006.
See #55824.

Built from https://develop.svn.wordpress.org/trunk@54151


git-svn-id: http://core.svn.wordpress.org/trunk@53710 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-content/themes/twentyfifteen/css/blocks.css

@@ -395,6 +395,10 @@ p.has-drop-cap:not(:focus)::first-letter {
 	vertical-align: baseline;
 }
 
+.is-style-outline .wp-block-button__link {
+	border: 2px solid;
+}
+
 .entry-content .wp-block-button:not(.is-style-outline) .wp-block-button__link {
 	border: 0;
 }

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-54150';
+$wp_version = '6.1-alpha-54151';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.