The solution to this is to change it in the App.xaml file – but going to the Application node (the very first one) and adding RequestedTheme="Light" to switch to the light theme or RequestedTheme="Dark" to switch to the dark theme.
This will have a massive impact on the overall appearance of your application!
A word of warning – you may also see the runtime property for this under App.Current.RequestedTheme and assume you can change it at runtime, however that will raise a NotSupportedException. What you can do is set it on start-up, so if you want to change it “dynamically” the user will need to restart the app for the change to be applied (Microsoft has a sample to show this).
Lastly an interesting tidbit from the documentation on this:
This property is ignored if the user is running in high contrast mode.For more posts in this series, see the series index.