This issue is best illustrated with an image:
As you can see above there is a grey border around the actual contents of the application.
What I've tried thus far is to modify the contents of .config/gtk-3.0/gtk.css according to the pointers given on a probably unrelated bug on CSD.
I've tried to select the area around the contents with gtk-inspector but to no avail.
Neither does applying, as seen above, a style removing margins, padding and borders to the background-class associated with the upper-most gtk element have any effect.
Having no experience at all with gtk development I'm unsure where to go from here.

