Index: src/wp-admin/js/updates.js
===================================================================
--- src/wp-admin/js/updates.js	(revision 41853)
+++ src/wp-admin/js/updates.js	(working copy)
@@ -1504,6 +1504,8 @@
 
 		$( 'body' ).addClass( 'modal-open' );
 		$modal.show();
+		/* FTP credentials dialog modal in not visible while trying to add a new theme as it was being blocked by body css visibility hidden */
+		$modal.css('visibility', 'visible');
 		$modal.find( 'input:enabled:first' ).focus();
 		$modal.on( 'keydown', wp.updates.keydown );
 	};