Fatal error: Uncaught Error: Call to undefined function wp_using_themes() in /wp-includes/template-loader.php:7
Stack trace:
#0 /wp-blog-header.php(19): require_once()
#1 /index.php(22): require('/wp-blog-header...')
#2 {main}
thrown in /wp-includes/template-loader.php on line 7