title: Changing background color in kivy
body: <p>I would like to change the background (black color) to a different color in kivy. But the Color specification in kv file is not recognized.</p>
  
  <p>main.py</p>
  
  <pre><code>from kivy.app import App
  from kivy.uix.boxlayout import BoxLayout
  from kivy.lang import Builder
  
  class MatrixCalcLayout(BoxLayout):
      def calculations(self):
          pass
  
  class ConfusionMatrixCalcApp(App):
      pass
  
  if __name__ == '__main__':
      ConfusionMatrixCalcApp().run()
  </code></pre>
  
  <p>ConfusionMatrixCalc.kv file</p>
  
  <pre><code>MatrixCalcLayout:
  
  &lt;MatrixCalcLayout&gt;:
      canvas:
          Color:
              rgba: 0.5, 0.5, 0.5, 0.5
      orientation: 'vertical'
      BoxLayout:
          Label:
      BoxLayout:
          Button:
      BoxLayout:
          Button:
      BoxLayout:
          Button:
  </code></pre>