As far as I am aware, nothing should prevent you modifying the content. However, I suppose it comes down to the timing of the loading of the iframe content and when the default styling is applied. Have you looked in the dev-tools/console to see which style is actually being applied? Do you have any other extensions on your page that set override any qlik styles?
Why not try writing out the object to the console (or step through your code) using console.log? That'll tell you whether the object you're looking for exists at the time you are trying to apply the style.