title: How to make opaque tutorial screen in flutter?
body: <p>I want to make tutorial screen that show to user at beginning. it's like below :</p>
  
  <p><a href="https://i.stack.imgur.com/IYrMM.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/IYrMM.jpg" alt="enter image description here"></a></p>
  
  <p>my specific question, how to make some certain elements will <strong>show normally</strong> and other are opaque ? </p>
  
  <p>also <code>the arrow</code> and text, how to make them <strong>point perfectly</strong> based on mobile device screen size  (<strong>mobile responsiveness</strong>) ?</p>