i *.h fil
under private:

 Dialog *window;

***************************************************
i *.cpp fil

i konstr.
window = new Dialog;

connect(window->button, SIGNAL(clicked()), this, SLOT(startGame()));
***************************
i startGame:

 window->setVisible(false);

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i keyPressed

 if (chosenNumber == randomNumber)
    {
        button->setStyleSheet("* { background-color: green }");
        window->setVisible(true);
    }