Index: wp-content/themes/twentyfifteen/css/editor-blocks.css
===================================================================
--- wp-content/themes/twentyfifteen/css/editor-blocks.css	(revision 55900)
+++ wp-content/themes/twentyfifteen/css/editor-blocks.css	(working copy)
@@ -900,6 +900,11 @@
 	padding: 1.6em 0;
 }
 
+/* Read More */
+.wp-block-read-more:hover {
+    color:#707070;
+}
+
 /* Latest Posts */
 
 .edit-post-visual-editor .wp-block-latest-posts.is-grid {