Add custom CSS style to Advanced Custom Fields WYSIWYG field

Example of adding styles from a theme. The code below works on frontend (if you are using acf_form()) as well as in the backend.

add_filter( 'tiny_mce_before_init', function($mce_init) {
  $content_css = get_stylesheet_directory_uri() . '/your-custom-css.css';

  if (isset($mce_init[ 'content_css' ])) {
    $mce_init[ 'content_css' ] = "{$mce_init['content_css']},{$content_css}";
  }

  return $mce_init;
});

Source