Source

File: wp-admin/menu.php

function _add_themes_utility_last() {
	// Must use API on the admin_menu hook, direct modification is only possible on/before the _admin_menu hook.
	add_submenu_page( 'themes.php', __( 'Theme Editor' ), __( 'Theme Editor' ), 'edit_themes', 'theme-editor.php' );
}