Register/unregister WordPress Gutenberg block styles

block-editor-scripts.js

/**
 * Scripts to run when in WordPress Gutenberg editor
 * 
 * Unregister any block styles we don't want user to be able to select
 * or register our own custom block styles.
 */
wp.domReady( () => {
    // Unregister any block styles we don't want user to be able to select
    wp.blocks.unregisterBlockStyle( 'core/quote', 'default' );
    wp.blocks.unregisterBlockStyle( 'core/quote', 'large' );
    wp.blocks.unregisterBlockStyle( 'core/button', 'default' );
    wp.blocks.unregisterBlockStyle( 'core/button', 'outline' );
    wp.blocks.unregisterBlockStyle( 'core/button', 'squared' );
    
    // Add custom block styles:
    wp.blocks.registerBlockStyle("core/button", {
      name: "mycustomstyle",
      label: "My Custom Style"
    });
} );

functions.php

<?php
function enqueue_block_editor_scripts() {
  wp_enqueue_script(
      'theme-block-editor-js',
      get_stylesheet_directory_uri() . '/js/blocks/block-editor-scripts.js',
      array( 'wp-blocks', 'wp-dom-ready', 'wp-edit-post' ), // specify dependencies to avoid race condition
      '1.0',
      true
  );
}
add_action('enqueue_block_editor_assets', 'enqueue_block_editor_scripts');
?>