It seems to me that it is common to augment the interface created by glade with a style resource file that modifies the look and feel of the interface. E.g. turning a standard GtkButton into a Big-Red button by a resource file. Currently there is no way of testing how the applicating will look like while still in glade3. I suggest adding somewhere in the interface the ability to load a resource file into a project. It would be even more fancy to be able to edit the resource file within glade, but that is possibly outside the scope of glade.