#include <QApplication>
#include <QLabel>

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

    QLabel *label = new QLabel();
    label->setText("abc");
    label->setStyleSheet("background-color: rgba(255, 247, 133, 0.8);"
                         "color: black;"
                         "font: 20pt Monospace");
    label->show();

    return app.exec();
}

TEMPLATE = app
TARGET = labeltest
QT += widgets
SOURCES += main.cpp

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
 <match target="font">
  <edit name="rgba" mode="assign">
   <const>rgb</const>
  </edit>
 </match>
</fontconfig>