Ok I get it , only Windows and fusion are available.
Noow it works.
Thank you really much.

You are welcome !

@mrjj
I try to use that line to change the color and the theme but it is not working:

    QApplication a(argc, argv);

    MainWindow w;
    w.show();

        a.setStyle("plastique");

Are you in qt4 or qt5?

If qt5 you have to use QStyleFactory

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

Have a look at this link from qt doc:

http://doc.qt.io/qt-5/qstylefactory.html#details

Depending or operating system styles are available to find which styles are available you can add this line :

qDebug() << QStyleFactory::keys();

For example for my operating system i have these available styles:

("Windows", "Motif", "CDE", "Plastique", "GTK+", "Cleanlooks")

    QApplication a(argc, argv);

    MainWindow w;
    w.show();

        a.setStyle("plastique");

How are you changing the color in qt designer?

You cannot. The dark style used in
Designer is not there as a selectable style when run outside.
Its a custom fusion based style.
You can re-create it with style sheets.

I think that you have to play with stretch property

you can put

4 for the first element
1 for the second element

for example

To understand what is stretch factor have a look at this link from qt doc:

http://doc.qt.io/qt-4.8/layout.html#stretch-factors

Hope this can help !