error in qml type converting while qml styling

[QtBeginnnnner]

first , I found two post and want to try their skinnable idea as follows :

https://stackoverflow.com/questions/36335481/dynamically-change-qml-theme-at-runtime/36399339#36399339

https://stackoverflow.com/questions/23518071/qt-5-styling-dynamically-load-qml-files/25866188#25866188

I almost copied their code , but failed and errors occurs:
Item {
id : base
...
property AbstractStyle currentStyle : Style.createStyleObject(base, "qrc:/StyleA.qml");

Text {
x: 100
y: 0
font.pixelSize: currentStyle.textSize
color: currentStyle.textColor
text: "Hello World"
}
}
while code running , it reports error showing "Unable to assign AbstractStyle_QMLTYPE_37 to AbstractStyle_QMLTYPE_0".

[QtBeginnnnner]

here is my code:
// AbstractStyle.qml
import QtQuick 2.0
QtObject {
property int textSize;
property color textColor;
}

// StyleA.qml
import QtQuick 2.0
AbstractStyle {
textSize : 20
textColor : "red"
}

// StyleB.qml
import QtQuick 2.0
AbstractStyle {
textSize : 50
textColor : "green"
}

//componentCreation.js
var component;
var sprite;
function createStyleObject(item, stylePath)
{
component = Qt.createComponent(stylePath);
if( component.status == Component.Ready ){
sprite = component.createObject(item);
console.log("[!!!OK!!!]componentCreation.js:" + stylePath + " component ready");
}
else
console.log("componentCreation.js:" + stylePath + " component not ready");
if (sprite === null)
console.log("componentCreation.js: error creating " + stylePath + " object");
else
return sprite;
}

//main.cpp
int main(int argc, char *argv[]) {
QGuiApplication app(argc, argv);

QQuickView *view = new QQuickView;
view->setResizeMode(QQuickView::SizeRootObjectToView);
view->setSource(QUrl(QStringLiteral("qrc:///main.qml")));
view->show();

return app.exec();

}

//main.qml
import "qrc:/styles/componentCreation.js" as Style
Item {
id : base
...
property AbstractStyle currentStyle : Style.createStyleObject(base, "qrc:/StyleA.qml");

Text {
x: 100
y: 0
font.pixelSize: currentStyle.textSize
color: currentStyle.textColor
text: "Hello World"
}
}

[QtBeginnnnner]

well , after some help and tests , it comes to conclusion:

this works in Qt 5.12.0 and above

property AbstractStyle currentStyle : Style.createStyleObject(base, "qrc:/StyleA.qml");

these two works in Qt 5.10.1 and above

property QtObject currentStyle : Style.createStyleObject(base, "qrc:/StyleA.qml");
property var currentStyle : Style.createStyleObject(base, "qrc:/StyleA.qml");