title: How to change the background image of a QMainWindow central widget?
body: <p>I'm trying to change the background image of the <code>QMainWindow</code> central widget. The <code>QMainWindow</code>'s background is fairly easy to change but I can't get the same result with it's central widget. What I tried is the code</p>
  
  <pre><code>self.mdi = QMdiArea()
  self.options_window = Options()
  self.central_widget = QStackedWidget()
  self.mdi.setStyleSheet("{background-image: url(ninja.png);}")
  self.setCentralWidget(self.central_widget)
  self.central_widget.addWidget(self.mdi)
  self.central_widget.addWidget(self.options_window)
  self.central_widget.setCurrentWidget(self.mdi)
  </code></pre>
  
  <p>I also tried with this one</p>
  
  <pre><code>self.mdi = QMdiArea()
  self.options_window = Options()
  self.central_widget = QStackedWidget()
  self.central_widget.setStyleSheet("{background-image: url(ninja.png);}")
  self.setCentralWidget(self.central_widget)
  self.central_widget.addWidget(self.mdi)
  self.central_widget.addWidget(self.options_window)
  self.central_widget.setCurrentWidget(self.mdi)
  </code></pre>
  
  <p>Could anyone give me a light in this problem?</p>