True if this theme was installed by the MaterialApp.
When an app uses the Navigator to push a route, the route's widgets
will only inherit from the app's theme, even though the widget that
triggered the push may inherit from a theme that "shadows" the app's
theme because it's deeper in the widget tree. Apps can find the shadowing
Theme.of(context, shadowThemeOnly: true) and pass it along
to the class that creates a route's widgets. Material widgets that push
routes, like PopupMenuButton and DropdownButton, do this.
final bool isMaterialAppTheme