GtkNotebook border style (GTK3) breaks fullscreen view in virt-manager

[TheBoctor]

The viewport of virt-manager is a GtkNotebook, so the border properties of notebook in your gtk-3.0/gtk.css file cause the picture in the application's fullscreen mode to be squashed and scaled incorrectly.

Is the embossed look necessary for the notebook widget? Is there a possible workaround or compromise to avoid this quirk? For the time being, I have commented out the necessary four lines on my machine.