Example code for a theme functions.php to enqueueing Gutenberg block CSS

enqueue-block-js-and-css-theme.php

<?php

/**
 * Enqueue block editor CSS
*/
function jsforwpblocks_editor_scripts() {

  // Make paths variables so we don't write em twice ;)
  $editorStylePath = '/assets/css/blocks.editor.css';

  // Enqueue optional editor only styles
  wp_enqueue_style(
    'jsforwp-blocks-editor-css',
    get_stylesheet_directory_uri() . $editorStylePath,
    ['wp-blocks'],
    filemtime( get_template_directory() . $editorStylePath )
  );

}
// Hook scripts function into block editor hook
add_action( 'enqueue_block_editor_assets', 'jsforwpblocks_editor_scripts' );


/**
 * Enqueue block editor CSS
*/
function jsforwpblocks_scripts() {

  // Make paths variables so we don't write em twice ;)
  $stylePath = '/assets/css/blocks.style.css';
  $frontendStylePath = '/assets/css/blocks.frontend.css';

  // Enqueue frontend and editor block styles
  wp_enqueue_style(
    'jsforwp-blocks-css',
    get_stylesheet_directory_uri() . $stylePath,
    [  'wp-blocks', 'wp-element', 'wp-components', 'wp-i18n' ],
    filemtime( get_template_directory() . $stylePath )
  );


  // Enqueue frontend only CSS
  if( !is_admin() ) {

    wp_enqueue_style(
      'jsforwp-blocks-frontend-css',
      get_stylesheet_directory_uri() . $frontendStylePath,
      [  'wp-blocks', 'wp-element', 'wp-components', 'wp-i18n' ],
      filemtime( get_template_directory() . $frontendStylePath )
    );
  }

}
// Hook scripts function into block editor hook
add_action( 'enqueue_block_assets', 'jsforwpblocks_scripts' );