title: How to add style via setStyleSheet() without losing orignal style in Qt?
body: <p>I Konw I can use <strong>setStyleSheet()</strong> to set style in Qt.But I encountered a problem,when I used <strong>setStyleSheet()</strong> twice first styles lost,which are set by first use of <strong>setStyleSheet()</strong>.</p>
  
  <p><strong>For exmaple</strong>,</p>
  
  <p><code>setStyleSheet("QLabel{color:red;}");</code></p>
  
  <p>…………</p>
  
  <pre><code>setStyleSheet("QLabel{border-image:url(……)}")
  </code></pre>
  
  <p><strong>When I set border-image,the red color property lost.</strong></p>
  
  <p>I tried to solve it by using </p>
  
  <p><code>setStyleSheet(styleSheet()+QString("QLabel{border-image:url(……)}"));</code> </p>
  
  <p>but it was the same that only the border-image property existed.</p>
  
  <p><strong>Must I add every style property when I use setStyleSheet(),although that I set it before.</strong></p>
  
  <p>Thanks for bearing my poor written English.Any tips will be appreciated.</p>