index.php

<?php

/**
 * Plugin Name: Gutenberg Block Styles
 * Plugin URI: https://github.com/Automattic/gutenberg-block-styles/
 * Description: A simple plugin to demonstrate how to add block styles to Gutenberg.
 * Version: 1.1
 * Author: Automattic
 */

/**
 * Register Custom Block Styles
 */
if ( function_exists( 'register_block_style' ) ) {
	function block_styles_register_block_styles() {
		/**
		 * Register stylesheet
		 */
		wp_register_style(
			'block-styles-stylesheet',
			plugins_url( 'style.css', __FILE__ ),
			array(),
			'1.1'
		);

		/**
		 * Register block style
		 */
		register_block_style(
			'core/paragraph',
			array(
				'name'         => 'blue-paragraph',
				'label'        => 'Blue Paragraph',
				'style_handle' => 'block-styles-stylesheet',
			)
		);
	}

	add_action( 'init', 'block_styles_register_block_styles' );
}