- Patch #19590 by Moshe: lazy initialization of the theme system.  Improves
  performance of pages that don't need the theme system (such as RSS feeds)
  and makes it easier to set a custom theme.

includes/common.inc

@@ -1905,7 +1905,4 @@ function clone($object) {
 // Initialize the localization system.
 $locale = locale_initialize();
 
-// Initialize the enabled theme.
-$theme = init_theme();
-
 ?>

includes/theme.inc

@@ -171,8 +171,12 @@ function list_theme_engines($refresh = FALSE) {
  *   An HTML string that generates the themed output.
  */
 function theme() {
-  global $theme;
-  global $theme_engine;
+  global $theme, $theme_engine;
+
+  if (!$theme) {
+    // Initialize the enabled theme.
+    $theme = init_theme();
+  }
 
   $args = func_get_args();
   $function = array_shift($args);