// 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');

    // Remove classic-themes CSS for backwards compatibility for button blocks.
    wp_dequeue_style('classic-theme-styles');
}, 20);