title: Godot Problematic CustomStyle, how can fix this?
body: <blockquote>
  <p>Godot version: 3.1 STABLE OS/device</p>
  <p>including version: WINDOWS/OS</p>
  </blockquote>
  <p>I’m making a movable window in my game, and I’ve used the <code>popup()</code> function. The fact is that I want to style it like a terminal, so I have made a custom style (<code>StyleBoxFlat</code>). The problem here is that the “title part” of the popup is becoming transparent. How I can fix this?</p>
  <p><strong>Illustration</strong>
  <a href="https://i.stack.imgur.com/RnCR0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RnCR0.png" alt="enter image description here" /></a></p>
  <p><strong>Solution from post to Godot Github tracker:</strong>
  <a href="https://i.stack.imgur.com/KuCyz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KuCyz.png" alt="enter image description here" /></a></p>