Index: wp-content/themes/twentynineteen/style-editor.css
===================================================================
--- wp-content/themes/twentynineteen/style-editor.css	(revision 55900)
+++ wp-content/themes/twentynineteen/style-editor.css	(working copy)
@@ -1605,3 +1605,8 @@
   padding-right: 0;
   width: 100%;
 }
+
+/* Post Author Block  */
+.wp-block-post-author__avatar img {
+    border-radius: 100%;
+}
\ No newline at end of file