#include <QApplication>

#include <QComboBox>
#include <QHBoxLayout>

QComboBox* createCombobox()
{
    const auto style = R"(
        QComboBox:open {
            border: 1px solid green;
        }
        QComboBox:closed {
            border: 1px solid red;
        }
    )";

    const auto comboBox = new QComboBox();
    comboBox->addItem("");
    comboBox->setStyleSheet(style);

    return comboBox;
}


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

    QWidget w;
    w.resize(640, 480);
    w.setLayout(new QHBoxLayout);
    w.layout()->addWidget(createCombobox());
    w.show();

    return a.exec();
}