I wrote a small project - system_changes (see Attachment: system_changes.pro, myapplication.h and main.cpp).

I override bool event(QEvent *) method in MyApplication class and log information about all event's types.

bool event(QEvent *event) override {
        qWarning() << event->type();

        return QApplication::event(event);
 }

In the Qt 5.5.1 there are messages

QEvent::Type(ApplicationPaletteChange)

QEvent::Type(ApplicationPaletteChange)

in the log after system style is being change to High Contrast and back.

But int the Qt 5.9.2 there are no such messages in log.

Why so?