QT 应用QSS时，没有生效？解决办法

#ifndef SHEET_H#define SHEET_H#include <QDialog>class QPushButton;class Sheet : public QDialog{   Q_OBJECTpublic:    Sheet(QWidget *parent = 0);private:    QPushButton *pushButton ;};#endif // SHEET_H

#include <QApplication>#include <QTextCodec>#include <QPushButton>#include <QFile>#include <QString>#include "sheet.h"Sheet::Sheet(QWidget *parent){    pushButton = new QPushButton("sheet",this);}int main(int argc,char **argv){    QApplication app(argc,argv);    QTextCodec::setCodecForTr(QTextCodec::codecForName("GB2312"));    Sheet *sheet = new Sheet;    QFile qss("C://style.qss");   //这个是绝对路径，，，用的是XP 系统,,编译能通过，就是没有效果，，    qss.open(QFile::ReadOnly);    qApp->setStyleSheet(qss.readAll());     qss.close();    sheet->setVisible(true);    return app.exec();}