Index: wp-content/themes/twentyseventeen/assets/css/blocks.css
===================================================================
--- wp-content/themes/twentyseventeen/assets/css/blocks.css	(revision 55900)
+++ wp-content/themes/twentyseventeen/assets/css/blocks.css	(working copy)
@@ -23,7 +23,7 @@
 [class^="wp-block-"]:not(.wp-block-gallery) figcaption {
 	font-style: italic;
 	margin-bottom: 1.5em;
-	text-align: left;
+	text-align: center;
 }
 
 .rtl [class^="wp-block-"]:not(.wp-block-gallery) figcaption {
Index: wp-content/themes/twentyseventeen/assets/css/editor-blocks.css
===================================================================
--- wp-content/themes/twentyseventeen/assets/css/editor-blocks.css	(revision 55900)
+++ wp-content/themes/twentyseventeen/assets/css/editor-blocks.css	(working copy)
@@ -463,7 +463,7 @@
 [class^="wp-block-"]:not(.wp-block-gallery) figcaption {
 	font-style: italic;
 	margin-bottom: 1.5em;
-	text-align: left;
+	text-align: center;
 }
 
 .rtl [class^="wp-block-"]:not(.wp-block-gallery) figcaption {