Index: wp-admin/includes/ajax-actions.php
===================================================================
--- wp-admin/includes/ajax-actions.php	(revision 40278)
+++ wp-admin/includes/ajax-actions.php	(working copy)
@@ -3507,7 +3507,7 @@
 		) );
 	}
 
-	$stylesheet = preg_replace( '/[^A-z0-9_\-]/', '', wp_unslash( $_POST['slug'] ) );
+	$stylesheet = preg_replace( '/[^A-z0-9._\-]/', '', wp_unslash( $_POST['slug'] ) );
 	$status     = array(
 		'delete' => 'theme',
 		'slug'   => $stylesheet,