QML - set default font

If you want set font for all items in ApplicationWindow. In main.cpp you must configure QFont and set it to QApplication.

    QApplication app(argc, argv);
    QFont font;
    font.setPixelSize(20);
    app.setFont(font);
}

Additional info here.

QtCreator highlight current line

If you want highlight current line in editor
Main menu: Tools > Options > Text Editor > Display > Highlight current line

QML - SystemPalette visible in all files

If you want use system colors create static variable in special singleton object.

// Style.qml
pragma Singleton
import QtQuick 2.11
QtObject {
     property SystemPalette palette: SystemPalette { colorGroup: SystemPalette.Active }
}

And then use it.

import QtQuick 2.0
Rectangle {
    color: Style.palette.highlight
}

Additional info here.