Dark theme is not automatically updated

[RodBarnes]

I've completed the workshop and implemented the AppThemes section, but the app doesn't pick up the change except when starting. Specifically, I can run the app -- it comes up in light theme styles. Then, change the device to Dark Theme -- but the app doesn't change. If I close the app and then re-open it, it picks up the theme change then and displays the dark theme styles. I've gone back through the section and compared to the "Finish" project but can't see anything I've done differently.