init method
ThemeServicePrefs initialization.
Must be called before using it.
Just get the shared preference instance with its static getter. We keep the handle to it in the instance so we do not need to get it more than once during the lifetime of this app.
The application only actually loads settings once when it starts, after that settings are in kept in memory and saved when changed, but never read back before app is started again.
Implementation
@override
Future<void> init() async {
// Get the SharedPreferences instance and assign it to our instance.
_prefs = await SharedPreferences.getInstance();
}