TestApp.vala

//Compile With: valac --thread --pkg gtk+-3.0 TestApp.vala
using Gtk;

public class TestBar : Gtk.HeaderBar{

    private unowned TestWindow main_window;
    public TestBar(TestWindow parent){
        get_style_context().add_class("my-bar");
        
        show_close_button = true;
        has_subtitle = true;
        
        title = "Test Application";
        subtitle = "CSS Doesn't Work :(";
        
        var test_button = new Gtk.Button.from_icon_name("gtk-add", Gtk.IconSize.LARGE_TOOLBAR);
        test_button.clicked.connect(() => {
            var ctx = get_style_context();
            message(ctx.has_class("my-bar") ? "TRUE" : "FALSE");
            message(@"background-color should be $(ctx.get_background_color(ctx.get_state()).to_string())"); // LIER! http://imgur.com/87kGWPC
        });
        pack_start(test_button);
        
    }

}

public class TestWindow : Gtk.Window{

    public static const string STYLE = """
    @define-color header-bg #2b2b2b;
    @define-color main-bg #8c8c8c;
    
    .my-bar {
        background-color: @header-bg;
    }
    
    .my-main-window{
        background-color: @main-bg;
    }
    """;

    public Gtk.CssProvider css_provider{get; private set;}
    private TestBar bar;
    
    public TestWindow(){
        get_style_context().add_class("my-main-window");
        
        init_css();
        
        set_size_request(800,600);
        
        bar = new TestBar(this);
        set_titlebar(bar);
        
        add(new Gtk.Label("Hello, World!"));
        
        this.destroy.connect(Gtk.main_quit);
    }
    
    private void init_css(){
        css_provider = new Gtk.CssProvider();
    
        try{
            css_provider.load_from_data(TestWindow.STYLE, -1);
            Gtk.StyleContext.add_provider_for_screen(get_screen(), css_provider, Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION);
        }catch(Error e){
            warning(@"Unable to load style information: [$(e.domain.to_string())]: $(e.message)");
        }
    }

}

public static int main(string[] args){
    Gtk.init(ref args);
    
    if (Environment.get_variable ("RTL") != null) {
        Gtk.Widget.set_default_direction (Gtk.TextDirection.RTL);
    }else{
        Gtk.Widget.set_default_direction (Gtk.TextDirection.LTR);
    }
    
    var window = new TestWindow();
    window.show_all();
    
    Gtk.main();

    return 0;
}