QPalette does not change the background of the button

I have the problem that the background of the button cannot be changed. Only the border is changed. I assumed that some attribute prevented this. But I think I'm wrong on that point. I've gone through the Qt documentation but can't find anything. I can only find examples on the internet that give me the same result. Is there a way to change the background?

Here is the code:

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    //ui->pushButton->setAttribute(Qt::WA_SetPalette);
    //ui->pushButton->setAttribute(Qt::WA_SetStyle);

    ui->pushButton->setAutoFillBackground(true);
    ui->pushButton->setAttribute(Qt::WA_ShowModal);
    QPalette pal = ui->pushButton->palette();

    //pal.setColor(QPalette::Base, Qt::cyan);
    //pal.setBrush(QPalette::Base, Qt::cyan);
    pal.setColor(QPalette::Button, Qt::cyan);
    pal.setBrush(QPalette::Button, Qt::cyan);
    
    ui->pushButton->setPalette(pal);

    ui->pushButton->update();

    //ui->pushButton->setAttribute(Qt::WA_NoSystemBackground, true);

//    setAutoFillBackground(true);
//    QPalette pal2 = palette();
//    pal2.setBrush(QPalette::Button, Qt::cyan);
//    pal2.setColor(QPalette::ButtonText, Qt::cyan);
//    QApplication::setPalette(pal2);
}

Here's the code I use to set the background color of a QPushButton (or any QAbstractButton):

// @param btn button to change colors of
// @param bc new background color for button
// @param optTextColor if non-NULL, the new color to use for the
//                button's text label.  If NULL, this function
//                will choose either black or white (whichever
//                it thinks will be more readable)
void SetButtonColor(QAbstractButton * btn, const QColor & bc, const QColor * optTextColor = NULL)
{
   QPalette p = btn->QWidget::palette();
   p.setColor(QPalette::Button, bc);

   const QColor fc = GetContrastingTextColor(bc);
   p.setColor(QPalette::Active,   QPalette::ButtonText, optTextColor?*optTextColor:fc);
   p.setColor(QPalette::Inactive, QPalette::ButtonText, optTextColor?*optTextColor:fc);
   p.setColor(QPalette::Disabled, QPalette::ButtonText, optTextColor?*optTextColor:MixColors(fc, Qt::lightGray, 0.5f));
   btn->setPalette(p);
}

// Returns either Qt::white or Qt::black, whichever will make for more readable text
// in front of the passed-in background color
QColor GetContrastingTextColor(const QColor & c)
{     
   const int darkThresh = 64;
   const int loneDelta  = 129;
   const int loneRed    = ((c.green()<darkThresh)&&(c.blue() <darkThresh)) ? loneDelta : 0;
   const int loneGreen  = 0;
   const int loneBlue   = ((c.red()  <darkThresh)&&(c.green()<darkThresh)) ? loneDelta : 0;
   return (std::max(c.red()-loneRed, std::max(c.green()-loneGreen, c.blue()-loneBlue)) >= 128) ? Qt::black : Qt::white;
}

// Returns a weighted average value between (v1) and (v2)
static int Mix(int v1, const int v2, float p) 
{
   return (int) ((((float)v2)*p)+(((float)v1)*(1.0f-p)));
}

// Returns a weighted average value between (c1) and (c2)
QColor MixColors(const QColor & c1, const QColor & c2, float p) 
{
   return QColor(Mix(c1.red(), c2.red(), p), Mix(c1.green(), c2.green(), p), Mix(c1.blue(), c2.blue(), p));
}