Earlier quoted context omitted.
This indeed. Custom UI widgets for Windows apps are really not necessary most of the time, and fitting in with the system theme is all you ought to aim for, most of the time. There are exceptions, for very rich dense UI, for UIs that need to be cross-platform, but if you're writing a Windows app, it should look and feel like a Windows app.
Or if you’re making games. All games seem to cherish a unique look even starting from the game menu or title screen.
Gone are the days of my friend accidentally having his volume cranked when he inserted the Red Alert 2 disc, not knowing it would blast out "WARINING! Military Software Detected!"