// Disable Gutenberg on the back end.

add_filter( 'use_block_editor_for_post', '__return_false' );

​

// Disable Gutenberg for widgets.

add_filter( 'use_widgets_block_editor', '__return_false' );

​

add_action( 'wp_enqueue_scripts', function() {

    // Remove CSS on the front end.

    wp_dequeue_style( 'wp-block-library' );

​

    // Remove Gutenberg theme.

    wp_dequeue_style( 'wp-block-library-theme' );

​

    // Remove inline global CSS on the front end.

    wp_dequeue_style( 'global-styles' );

}, 20 );