Index: wp-content/themes/twentyfifteen/css/editor-blocks.css
===================================================================
--- wp-content/themes/twentyfifteen/css/editor-blocks.css	(revision 53731)
+++ wp-content/themes/twentyfifteen/css/editor-blocks.css	(working copy)
@@ -344,8 +344,8 @@
 .edit-post-visual-editor a,
 .editor-block-list__block a,
 .wp-block-freeform.block-library-rich-text__tinymce a {
-	border-bottom: 1px solid #333;
-	color: #333;
+	border-bottom: 1px solid currentColor;
+	color: currentColor;
 	text-decoration: none;
 }
 
Index: wp-content/themes/twentyfifteen/style.css
===================================================================
--- wp-content/themes/twentyfifteen/style.css	(revision 53731)
+++ wp-content/themes/twentyfifteen/style.css	(working copy)
@@ -648,7 +648,7 @@
  */
 
 a {
-	color: #333;
+	color: currentColor;
 	text-decoration: none;
 }