fix 404 pages for derivative themes by pretending we've found a module called '404'. This way all the correct theme initialisation stuff will happen.

index.php

@@ -240,6 +240,9 @@ if(strlen($a->module)) {
 		$a->page['content'] = replace_macros($tpl, array(
 			'$message' =>  t('Page not found.' )
 		));
+		// pretend this is a module so it will initialise the theme. 
+		$a->module = '404';
+		$a->module_loaded = true;
 	}
 }