add_filter(‘tiny_mce_before_init’, function ($init) {

$css = ‘
body#tinymce.cx_wysiwyg {
color: #222 !important;
background-color: #fff !important;
}
‘;

if (!empty($init[‘content_style’])) {
$init[‘content_style’] .= $css;
} else {
$init[‘content_style’] = $css;
}

return $init;
});