AFAIK, there's no stylesheet for that button icon.
A cheating way is to set the icon property by the object name written in the Qt source code.
QToolButton#qt_toolbar_ext_button{qproperty-icon:url(your_custom_image_url)}
But the icon will be reset when toolbar's orientation changed, so if your toolbar's orientation may change, you need to connect to orientationChanged signal and set the stylesheet again.
Note: The above method may become invalid if Qt changes the source code in the future.
Another way is to implement a QProxyStyle and set it to be the application's style.
class MyProxyStyle : public QProxyStyle
{
public:
QIcon standardIcon(QStyle::StandardPixmap standardIcon, const QStyleOption *option = nullptr,
const QWidget *widget = nullptr) const override
{
if(standardIcon == QStyle::SP_ToolBarHorizontalExtensionButton)
return QIcon("your_custom_image_url_for_horizontal");
else if (standardIcon == QStyle::SP_ToolBarVerticalExtensionButton)
return QIcon("your_custom_image_url_for_vertical");
return QProxyStyle::standardIcon(standardIcon, option, widget);
}
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
a.setStyle(new MyProxyStyle);
...
return a.exec();
}
Note: This will work on all toolbars in the application.
