#include <QtWidgets/QApplication>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QComboBox>
#include <QtWidgets/QVBoxLayout>


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

	QMainWindow main_window;
	main_window.setGeometry(500, 500, 500, 300);

	// Make central widget for main window
	QWidget* central_widget = new QWidget(&main_window);
	central_widget->setLayout(new QVBoxLayout(central_widget));
	main_window.setCentralWidget(central_widget);


	QWidget* inner_widget = new QWidget(central_widget);
	central_widget->layout()->addWidget(inner_widget);
	inner_widget->setLayout(new QVBoxLayout(inner_widget));

	QComboBox* combobox = new QComboBox(inner_widget);
	combobox->addItem("a");
	combobox->addItem("b");
	inner_widget->layout()->addWidget(combobox);

	combobox->setPalette(QPalette(QColor(0, 200, 0)));

	central_widget->setStyleSheet("QComboBox { background-color: rgb(100, 100, 100); border: 0px; } ");

	main_window.show();
	return app.exec();
}