#include <QApplication>
#include <QPalette>
#include <QVBoxLayout>
#include <QWidget>

void setThemePalette()
{
    QPalette themePalette;
    themePalette.setColor(QPalette::Window, Qt::red);
    QApplication::setPalette(themePalette);
}

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

    /*

    Expected result:

      +--------------------+
      | w1 (Qt::green)     |
      |  +--------------+  |
      |  | w2 (Qt::red) |  |
      |  +--------------+  |
      |                    |
      +--------------------+

    */

    QWidget w1;
    QPalette p1;
    p1.setColor(QPalette::Window, Qt::green);
    w1.setPalette(p1);

    auto w2 = new QWidget;
    QPalette p2;
    p2.setColor(QPalette::Window, Qt::red);
    w2->setPalette(p2);
    w2->setAutoFillBackground(true);
    w2->setMinimumSize(100, 100);

    auto layout = new QVBoxLayout(&w1);
    layout->addWidget(w2);
    w1.show();

    return a.exec();
}