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");