Qt is not qute

my qt program's gui looks like normal...

need to add style header in the including file:

#include
#include
#include "mainwindow.h"

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
a.setStyle(new QPlastiqueStyle); //this command must be in front of ”return a.exec()”
return a.exec();
}