Comment 4 for bug 1340392

For this one, we want to assign https://valadoc.org/gtk+-3.0/Gtk.STYLE_CLASS_FLAT.html

We do this by getting the style context of the widget and adding a style class like so:

widget.get_style_context ().add_class (foo);

where foo is your class name and in this case we want to use the constant Gtk.STYLE_CLASS_FLAT