title: Can't set CSS to specified widget in GTK+
body: <p>I'm using Vala with GTK+ and now I'm trying to add custom CSS to specified widget.
  I can add fe. backgroudn to GtkWidget but not for #sidebar</p>
  
  <pre><code>#sidebar { //It doesn't work                                                           
      color: white;                                                                
  }                                                                       
  
  GtkWindow {  // It works                                                                    
      background-color: red;                                                                        
  }   
  </code></pre>
  
  <p>I'm adding class to widget like that: </p>
  
  <pre><code>sidebar = new Gtk.Label("Hello");
  sidebar.set_name("sidebar");
  </code></pre>
  
  <p>And it's changes color to GtkWindow, but not for this label.</p>
  
  <p>Any ideas?</p>