title: Gtk.CssProvider.load_from_data TypeError: Item 0: Must be number, not str
body: <p>I was getting this error when running my gtk project from a terminal with <code>python3 myapp.py</code>, but not when launching my app via Sublime-Text-3's python build system! How weird.</p>
  
  <pre><code>TypeError: Item 0: Must be number, not str
  </code></pre>
  
  <hr>
  
  <p>The code at fault was essentially:</p>
  
  <pre><code>color_scheme = {'fg_color':'#ffffff', 'bg_color': '#000000',}
  css = """
      GtkLabel {
          color: %(fg_color)s;
          background: %(bg_color)s;
      }
  """
  
  # ...
  
  cssprovider = Gtk.CssProvider()
  css_data = css % color_scheme
  cssprovider.load_from_data(css_data)
  </code></pre>