frames: Add default-decoration CSS class

Add this CSS class both to the header bar itself, since it is what
actually contains the window controls, and to MetaFrameHeader too,
since it's what's directly attached to the window.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2740>

src/frames/meta-frame-header.c

@@ -115,6 +115,8 @@ meta_frame_header_init (MetaFrameHeader *content)
   GtkWidget *header_bar;
 
   header_bar = gtk_header_bar_new ();
+  gtk_widget_add_css_class (header_bar, "titlebar");
+  gtk_widget_add_css_class (header_bar, "default-decoration");
   gtk_widget_insert_before (header_bar, GTK_WIDGET (content), NULL);
 }