remove background only on main window's titlebar

data/resources/style.css

@@ -60,11 +60,11 @@ entry selection{
   color: shade(@foreground_color, 0.9);
 }
 
-window headerbar.titlebar {
+window.main-window headerbar.titlebar {
   background: none;
   border: none;
 }
 
-window headerbar.titlebar button.close {
+window.main-window headerbar.titlebar button.close {
   margin-right: 6px;
 }

src/window.rs

@@ -69,6 +69,7 @@ impl ObjectImpl for WindowPrivate {
         if PROFILE == "Devel" {
             self_.get_style_context().add_class("devel");
         }
+        self_.get_style_context().add_class("main-window");
         self_.set_icon_name(Some(APP_ID));
         // Keyboard Shortcuts
         let builder = gtk::Builder::from_resource("/org/gnome/design/Contrast/shortcuts.ui");