Edit File: custom-background.php
<?php /** * Custom background script. * * This file is deprecated, use 'wp-admin/includes/class-custom-background.php' instead. * * @deprecated 5.3.0 * @package WordPress * @subpackage Administration */ // Don't load directly. if ( ! defined( 'ABSPATH' ) ) { die( '-1' ); } _deprecated_file( basename( __FILE__ ), '5.3.0', 'wp-admin/includes/class-custom-background.php' ); /** Custom_Background class */ require_once ABSPATH . 'wp-admin/includes/class-custom-background.php';