title: navigation buttonItem can not show and the navigationItem background color is not very clear
body: <p>why the <code>navigationbar</code> can not show leftButton and rightButton, but if I click there, it has response. And I set the background color blue, but it looks not very clear, like something covers on the <code>navigationItem</code>.</p>
  
  <p><img src="https://i.stack.imgur.com/EL5Rc.jpg" alt=""></p>