Create your normal derivation

1. Create C++ and .h files
2. Do your stuff

Reimplement the paintEvent with:

void YourClass::paintEvent(QPaintEvent *event){
    Q_UNUSED(event)

    QStyleOption option;
    option.init(this);

    QPainter painter(this);

    style()->drawPrimitive(QStyle::PE_Widget, &option, &painter, this);
}

Now YourClass will work as valid style selector