app = QtGui.QApplication(sys.argv)
w = QtGui.QComboBox()
w.setEditable(True)
c = QtGui.QCompleter(['Hello', 'World'])
c.setCompletionMode(QtGui.QCompleter.UnfilteredPopupCompletion)
c.popup().setStyleSheet('background-color: yellow')
w.setCompleter(c)
w.show()
sys.exit(app.exec_())
Styling the popup of a QCompleter in PyQt