Banner Designer isn't able to parse default app set
Importing data/assets/org.gnome.Software.Featured.xml silently fails. I don't think this should be an explicit action even. The banner designer should just load the banner set and save on valid CSS parse, because its purpose is to design banners for Software.
This would compose a great Builder workflow of editing/adding new banners, with the biggest stumbling block being asset installation (the need to edit the meson build file, run a build, restart Designer and only then being able to reference the assets). Small steps though...
CC @bertob