This clarifies a few details about icons and icon themes on Mac and MS Windows, calling attention to the fact that icon themes can be available as usual and that embedded icon resources are thus not the only way applications can have access to such themes.
While not directly related I think this would also be a good place to draw attention to the fact that the usual
app.setWindowIcon(QIcon::fromTheme(appName)));
can have the opposite effect on those systems and should thus be avoided or use the QIcon::fromTheme() fallback argument
app.setWindowIcon(QIcon::fromTheme(appName, app.windowIcon()));