Hi Kevin,
You can use styling to change some of the properties of the decorators, for example stroke, fill and so on. The decorator shape, however, is currently not styleable and that is why you should iterate through all decorators and change the shape of the decorator locally. As you have mentioned in your post, this is demonstrated in the Diagram Examples and more specifically in the example "Document Object Model -> Decorators -> Show/Hide Subtree Decorator".
foreach (NShowHideSubtreeDecorator decorator in document.Descendants(new NInstanceOfTypeFilter(typeof(NShowHideSubtreeDecorator)), -1))
{
decorator.Background.Shape = shape;
}
document.SmartRefreshAllViews();