title: GTK - How to style a button in headerbar?
body: <p>I want to set color like blue and red in a application, is something like 
  <a href="http://i.stack.imgur.com/c9bn8.jpg" rel="nofollow">this image</a>.</p>
  
  <p>I imagine that I can do it with css styles, but if is there other way much more simple like set a property or a class.</p>
  
  <p>I use GTK 3.12 and Python, but you can show me examples in other lenguages and I can traduce it.</p>