QGuiApplication::setWindowIcon(...) only takes effect in the first two times.

QGuiApplication app(argc, argv);

// set window icon

// first time(it work fine)
app.setWindowIcon(QIcon(QPixmap(app.styleHints()->colorScheme() == Qt::ColorScheme::Dark ?
	":/resources/onnx-white.svg" : ":/resources/onnx.svg")));

// only work when first switched dark or light theme
QObject::connect(app.styleHints(), &QStyleHints::colorSchemeChanged, &app,
	[&app](Qt::ColorScheme colorScheme) {
		qDebug() << "Color Scheme Changed: " << colorScheme;
		if (colorScheme == Qt::ColorScheme::Dark)
			app.setWindowIcon(QIcon(QPixmap(":/resources/onnx-white.svg")));
		else if (colorScheme == Qt::ColorScheme::Light)
			app.setWindowIcon(QIcon(QPixmap(":/resources/onnx.svg")));
	});