int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QFile File("://stylesheet.qss");
    File.open(QFile::ReadOnly);
    QString StyleSheet = QLatin1String(File.readAll());
    qApp->setStyleSheet(StyleSheet);

    Theme * themeInstance;

    QApplication::setStyle(QStyleFactory::create("Fusion"));

    QPalette p;
    p.setColor(QPalette::Window, themeInstance->getCustomColor());
    qApp->setPalette(dp);

    return a.exec();
}

class Theme : public QWidget
{
    Q_OBJECT
    Q_PROPERTY(QColor customColor READ custombgColor WRITE setCustomColor DESIGNABLE true)
public:
    Theme(QWidget *parent = nullptr);

    QColor customColor() const { return m_customColor; };
    void setCustomColor(const QColor &c) { m_customColor = c; };
private:
    QColor m_customColor;
protected:
};