QDockWidget titlebars not dark-light mode updated on macOS if stylesheet set

On macOS, if you switch from dark to light or light to dark mode while a Qt app is open and that app loads a qss stylesheet and uses QDockWidgets, the titlebar of the QDockWidgets never get properly updated.

This only happens when the app loads a stylesheet.

I have created a standalone test case from your official dockwidgets example.

It uses qmake to build.

unzip dockwidget_bug.zip
cd dockwidget_bug
qmake
make
open ./dockwidgets.app

When open, simply go to macOS System Settings Appearance and change light to dark mode or visa-versa.

You will see the QDockwidget titlebars are not properly repainted.

The only way around this I have found is to use something like the following in the QDockWidget subclass to override the paintEvent and force the titlebar to be painted properly:

```

void BookBrowser::paintEvent(QPaintEvent *event)
{
    QStylePainter painter(this);
    if (isFloating()) {
        QStyleOptionFrame options;
        options.initFrom(this);

#ifdef Q_OS_MAC
        // This is needed for Qt6 but works on Qt5 as well
        options.palette = QApplication::palette();
#endif

        painter.drawPrimitive(QStyle::PE_FrameDockWidget, options);
    }
    QStyleOptionDockWidget options;
    initStyleOption(&options);
    options.title = windowTitle();

#ifdef Q_OS_MAC
    // This is needed for Qt6 but works on Qt5 as well
    options.palette = QApplication::palette();
#endif

    painter.drawControl(QStyle::CE_DockWidgetTitle, options);
}

```