Index: ajax-actions.php
===================================================================
--- ajax-actions.php	(revision 58269)
+++ ajax-actions.php	(working copy)
@@ -4289,7 +4289,7 @@
 		);
 	}
 
-	$stylesheet = preg_replace( '/[^A-z0-9_\-]/', '', wp_unslash( $_POST['slug'] ) );
+	$stylesheet = sanitize_file_name( wp_unslash( $_POST['slug'] ) );
 	$status     = array(
 		'update'     => 'theme',
 		'slug'       => $stylesheet,