#include <QApplication>
#include <QDockWidget>
#include <QLabel>
#include <QHBoxLayout>
#include <QMainWindow>

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::AA_UseStyleSheetPropagationInWidgetStyles);
    QApplication a(argc,argv);
    QMainWindow mw;
    mw.setGeometry(50,50,600,400);
    mw.setContextMenuPolicy(Qt::ContextMenuPolicy::CustomContextMenu);

    a.setStyleSheet("QMainWindow { color: red }");

    QLabel *l = new QLabel("Test", &mw);
    mw.show();
    QObject::connect(&mw, &QWidget::customContextMenuRequested, l, [&mw, l] (const QPoint&)
        {
            delete l;
            QLabel *ll = new QLabel("Test 2", &mw);
            ll->show();
            //  workaround to force palette propagation:
           //    ll->setStyleSheet("background-color: blue");
           //    ll->setStyleSheet("");
        });
    return a.exec();
}