Add CSS to ACF WYSIWYG Editor

Example of adding a CSS file from your theme to the WYSIWYG editor of the Advanced Custom Fields plugin. The code below also works on the frontend if you use the acf_form() function.

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;
});