How do I reference a StaticResource in code-behind?

The page-level Resources object has the ability to find local, app-level, static, and theme resources. This means you simply do this:

foo2.Style = this.Resources["NavigationBackButtonNormalStyle"] as Style;

Best of luck!

During design-time, it seems that trying to resolve a "system resource" using Resources[key] will fail to find the resource and will return null. For example, to get the base Style for a TextBox using Resources[typeof(TextBox)] will return null.

Instead, use TryFindResource(key) since this will first try Resources[key] and then will otherwise try searching through the "system resources" and will return what you're looking for (as per MSDN and Reference Source).

In other words, try this instead:

var style = Application.Current.TryFindResource(key) as Style;