change theme with radio button Qwidget in Qt C++

I want to change my application Theme with QRadioButton in the QWidget application. can anyone help me? I have a Setting class that has some settings and one of them is changing the theme between dark and light. it is a C++ app with OpenGL.

void Settings::on_DarkTheme_clicked()
{
    
}

void Settings::on_lightTheme_clicked()
{
    
}

what should I have to write in these 2 methods?

I used .qss file like this

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


QSurfaceFormat fmt;
fmt.setVersion(3,3);
fmt.setProfile(QSurfaceFormat::CoreProfile);
fmt.setSamples(8);
fmt.setOption(QSurfaceFormat::StereoBuffers);
fmt.setSwapBehavior(QSurfaceFormat::DoubleBuffer);
QSurfaceFormat::setDefaultFormat(fmt);

loadSettings();

App app(argc, argv);

QFile stylesheetFile("./Combinear.qss");
stylesheetFile.open(QFile::ReadOnly);
QString stylesheet = QLatin1String(stylesheetFile.readAll());
app.setStyleSheet(stylesheet);
MainWindow w;
w.show();
const auto returnValue = app.exec();
App::setFinished();
return returnValue;
}

but it didn't Work and I'm confused

First of all, I add a radioButton in my mainwindow.ui .

this is my mainwindow.h :

#ifndef MAINWINDOW_H

#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui
{
class MainWindow;
}
QT_END_NAMESPACE

class MainWindow: public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);

    ~MainWindow();

signals:
    void  changedDark();

    void  changedLight();

private slots:
    void  on_radioButton_clicked(bool checked);

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

and my mainwindow.cpp :

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

MainWindow::MainWindow(QWidget *parent):
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

MainWindow::~MainWindow()
{
    delete ui;
}

void  MainWindow::on_radioButton_clicked(bool checked)
{
    if (checked)
    {
        ui->radioButton->setText("Dark");
        emit  changedDark();
    }
    else
    {
        ui->radioButton->setText("Light");
        emit  changedLight();
    }
}

and my main.cpp:

#include "mainwindow.h"

#include <QApplication>
#include <QFile>

int  main(int argc, char *argv[])
{
    QApplication  a(argc, argv);
    MainWindow    w;

    w.show();

    QObject::connect(&w, &MainWindow::changedDark, &a, [&]
    {
        /**
         * Load the application style
         */
        QFile dark_styleFile(":/theme/themes/MaterialDark.qss");

        dark_styleFile.open(QFile::ReadOnly);

        /**
         * Apply the loaded stylesheet
         */
        QString style(dark_styleFile.readAll());
        a.setStyleSheet(style);
    });

    QObject::connect(&w, &MainWindow::changedLight, &a, [&]
    {
        /**
         * Load the application style
         */
        QFile light_styleFile(":/theme/themes/Ubuntu.qss");

        light_styleFile.open(QFile::ReadOnly);

        /**
         * Apply the loaded stylesheet
         */
        QString style(light_styleFile.readAll());
        a.setStyleSheet(style);
    });

    return a.exec();
}

This is my result:

you can clone it from this repo:

https://github.com/parisa-hr/qt-theme-example