title: How to use CssStyleProvider in Gtkmm3
body: <p>I'm programming in c++ with gtkmm3. I want to change some fonts in my program. I read all the things about CssStyleProvider, StyleContext in gtkmm documentation and also in gtk+ documentaion but i couldn't make it work and couldn't find any tutorial about that. I'm trying like that</p>
  
  <pre><code>Glib::ustring data;
  data="GtkMenuBar, GtkMenuItem {font-name: Sans 6}"; /*tried with semi-colon, too*/
  Glib::RefPtr&lt;Gtk::CssProvider&gt; asd = Gtk::CssProvider::create();
  Glib::RefPtr&lt;Gtk::StyleContext&gt; asd2 = Gtk::StyleContext::create();
  asd-&gt;load_from_data(data);
  asd2-&gt;add_provider(asd, 0); /*also tried different priorities*/
  </code></pre>
  
  <p>also tried like that</p>
  
  <pre><code>Glib::ustring data;
  data="GtkMenuBar, GtkMenuItem {font-name: Sans 6}"; /*tried with semi-colon, too*/
  Glib::RefPtr&lt;Gtk::CssProvider&gt; asd = Gtk::CssProvider::create();
  Glib::RefPtr&lt;Gdk::Screen&gt; screen;
  Glib::RefPtr&lt;Gtk::StyleContext&gt; asd2 = Gtk::StyleContext::create();
  asd-&gt;load_from_data(data);
  (mainWindow.get())-&gt;get_property("screen", screen);
  asd2-&gt;add_provider(screen, asd, 0); /*also tried different priorities*/
  </code></pre>
  
  <p>And I tried these before running my main window and after running it but i have no luck. Any help is appreciated.</p>