No need for require_once in these files.

themes.php

@@ -8,13 +8,13 @@
 /**
  * get some globals
  */
-require_once './libraries/common.inc.php';
+require './libraries/common.inc.php';
 
 /* Theme Select */
 $path_to_themes = $cfg['ThemePath'] . '/';
 
 /* set language and charset */
-require_once './libraries/header_http.inc.php';
+require './libraries/header_http.inc.php';
 
 /* HTML header */
 $page_title = 'phpMyAdmin - ' . __('Theme / Style');