Index: wp-content/themes/twentysixteen/css/editor-blocks.css
===================================================================
--- wp-content/themes/twentysixteen/css/editor-blocks.css	(revision 55900)
+++ wp-content/themes/twentysixteen/css/editor-blocks.css	(working copy)
@@ -305,6 +305,10 @@
 	padding: 0 0 0 1.263157895em;
 }
 
+.editor-styles-wrapper .wp-block-quote {
+    border-color: currentColor;
+}
+
 .wp-block-quote:not(.is-large):not(.is-style-large).alignleft,
 .wp-block-quote:not(.is-large):not(.is-style-large).alignright {
 	border-left: 0;