Ok, I have reported the bug here.
I’ve also removed the references to the gtk-common-themes from the snapcraft.yaml but I still get the error regarding gnome-3-26-1604 when I try to start the app:
$ gwe
You need to connect this snap to the gnome platform snap.
You can do this with those commands:
snap install gnome-3-26-1604
snap connect gwe:gnome-3-26-1604 gnome-3-26-1604
(the '3-26-1604' number defines the platform version and might change)
Should I report this other issue somewhere else? @popey @zyga-snapd @kenvandine any advice?