title: GTK3: listen to theme changes
body: <p>I'm looking for a way, on a PyGObject application, to be notified when there is a theme change.</p>
  <p>I need this notification because I'm plotting a graph using <code>matplotlib</code> and I'm setting as text color for the Graph the standard label color of the current GTK3 theme:</p>
  <pre><code>temp_label = Gtk.Label()
  scrolled_window.add(temp_label)
  text_color = rgba_to_hex(temp_label.get_style_context().get_color(Gtk.StateType.NORMAL))
  </code></pre>
  <p>But, when the user switches theme, I need to fetch the new label color and redraw the graph.</p>
  <p>Currently, going from a dark to a light theme, makes the labels unreadable:
  <a href="https://i.stack.imgur.com/zbSdo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zbSdo.png" alt="readable labels" /></a>
  <a href="https://i.stack.imgur.com/auLr6.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/auLr6.png" alt="unreadable labels" /></a></p>