background can not be set transparent on Lubuntu LXQT desktop

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QWidget* tab_content = new QWidget();
    tab_content->setAttribute(Qt::WA_OpaquePaintEvent); //makes the widget transparent

    QTabWidget* tw = new QTabWidget();
    tw->addTab(tab_content, "Example");
    tw->setStyleSheet("QTabBar::tab { background: transparent; }"); //makes the header transparent

    QWidget w;
    w.setAttribute(Qt::WA_TranslucentBackground); //makes the window transparent
    w.setLayout(new QVBoxLayout());
    w.layout()->addWidget(tw);
    w.show();

    return a.exec();
}