Change browser background colour javascript not working

The page Storyline 3: JavaScript Best Practices and Examples says that the following code inserted into the javascript window and run on timeline start will change the background colour of the browser from its defaukt white to whatever colour is stated.

parent.document.body.style.backgroundColor = "#990000";

Unfortunately it's not working for me. I have tried a couple of browsers. I have uploaded the course to a server and tried from there. No joy. Any suggestions appreciated.