title: Format Kivy popup to eliminate popup background
body: <p>we're using an image as a popup in our kivy app but can't seem to get the popup's background to fit the size of the image. The resulting popup has the image with a dark gray border around the left side, top, and right side of the image. Can someone help us eliminate the popup's background so that only the image and the semi-transparent filter are visible?</p>
  
  <p>Main.py code:</p>
  
  <pre><code>popup = Popup(title="",
                    content=Image(source='img/popup'),
                    auto_dismiss=True,
                    size_hint=(None, None),
                    size=(400, 146),
                    separator_height=0
                    )
  </code></pre>
  
  <p>Thanks</p>