Problem to add CSS in InfoBox after upgrade from 1.50 to 1.71

I just done the upgrade from 1.50 to 1.71.

I was using this code to add a css in the Iframe, but it is not working anymore.

    viewer.infoBox.frame.addEventListener('load', function () {
                    var customCssLink = viewer.infoBox.frame.contentDocument.createElement('link');
                    customCssLink.href = relativeRootUrl + '/SomePath/CSS/CesiumPopUp.css';
                    customCssLink.rel = 'stylesheet';
                    customCssLink.type = 'text/css';
                    viewer.infoBox.frame.contentDocument.head.appendChild(customCssLink);});

Have you some tips to help me?