In the last Article you created a new App ThemeDemoApp. Let’s refer to this App to understand the Theme.
Currently the Activity class extends ActionBarActivity. This class is used in conjunction with Theme.AppCompat themes to add action bar to apps that support API levels b/w 7 and 10. If you use this as the superclass you have to use one of these Themes or your App won’t run.
For now you can refer to the Manifest and style XML files to see how theme is referred to.
Source Code available here