qt-creator/share/qtcreator/qmldesigner/propertyeditor/Qt/applybutton.css

QPushButton {
    border-image: url(:/qmldesigner/images/apply.png) 3;
    border-width: 3;
 }
 
  QPushButton:hover {
    border-image: url(:/qmldesigner/images/applybright.png) 3;
 }