web/app/mu-plugins/register-theme-directory.php
<?php
/**
* Plugin Name: Register Theme Directory
* Plugin URI: https://github.com/roots/bedrock/
* Description: Register default theme directory
* Version: 1.0.0
* Author: Roots
* Author URI: https://roots.io/
* License: MIT License
*/
if ( ! defined( 'WP_DEFAULT_THEME' ) ) {
register_theme_directory( ABSPATH . 'wp-content/themes' );
}