Issue #3446: Updated Gtk3.18 theme

Harcoded style has been reintroduced for Gtk < 3.20 only ; a cleanup
will have to be done once Gtk3.18 support will be dropped.

rtgui/preferences.cc

@@ -56,6 +56,11 @@ Preferences::Preferences  (RTWindow *rtwindow)
     set_default_size (options.preferencesWidth, options.preferencesHeight);
 
     Gtk::Box* mainBox = get_content_area ();
+//GTK318
+#if GTK_MAJOR_VERSION == 3 && GTK_MINOR_VERSION < 20
+    mainBox->set_spacing(8);
+#endif
+//GTK318
     //set_has_separator (false);
 
     Gtk::Notebook* nb = Gtk::manage (new Gtk::Notebook ());