qt dialog 弹出后背景变暗透明

int width1 = this->width();

    int height1 = this->height();

    QWidget *m_mask = new QWidget(this);

    m_mask->resize (width1, height1);

    m_mask->move (0,0);

    m_mask->setWindowFlags(Qt::FramelessWindowHint);

    m_mask->setAttribute(Qt::WA_StyledBackground);

    m_mask->setStyleSheet("");

    int width2 = 520;

    int height2 = 280;

    QDialog *m_widget = new QDialog(this);

    m_widget->setWindowFlags(Qt::FramelessWindowHint);

    m_widget->setAttribute(Qt::WA_StyledBackground);

    m_widget->resize(width2,height2);

    m_widget->move((width1-width2)/2,(height1-height2)/2);

    QLabel *bg = new QLabel(m_widget);

    bg->resize(width2,height2);

    //bg->move((width1-width2)/2,(height1-height2)/2);

    bg->setPixmap(QPixmap(":/image_normal/bg_tip_warn.png"));

    QFont font;

    font.setFamily(FONT_B);

    font.setPixelSize(30);

    QLabel *title = new QLabel(bg);

    //title->setAlignment(Qt::AlignCenter);

    title->setGeometry(251,14,64,42);

    title->setText(QStringLiteral("提示"));

    title->setFont(font);

    title->setStyleSheet("color:rgb(255,255,255);"> transparent;");

    //title->setStyleSheet("QLabel{color:rgb(255,255,255);"> transparent;}");

    font.setFamily(FONT_M);

    font.setPixelSize(24);

    QLabel *lab_text = new QLabel(bg);

    lab_text->resize(width2,99);

    lab_text->move(0,85);

    lab_text->setAlignment(Qt::AlignHCenter);

    lab_text->setText(text);

    lab_text->setFont(font);

    lab_text->setWordWrap(true);

    title->setStyleSheet("color:rgb(255,255,255);");

    //lab_text->setStyleSheet("QLabel{color:rgb(38,38,38);"> transparent;}");

    //m_widget->show();

    QPushButton* btn_ok=new QPushButton(bg);

    btn_ok->resize(150,50);

    btn_ok->move(188,195);

    btn_ok->setFont(font);

    btn_ok->setStyleSheet("QPushButton{color:rgb(255,255,255);}");

    btn_ok->setText(QStringLiteral("确定"));

    connect(btn_ok,&QPushButton::clicked,m_widget,&QDialog::accept);

    m_mask->show();

    int nRes = m_widget->exec();

    if (nRes == QDialog::Accepted)

    {

        m_widget->deleteLater();

        m_mask->deleteLater();

    }