There is one pattern set/palette per deck, but as demonstrated by the above materials it can be manipulated on the fly; it's a matter of scripting to give cards their own palette. Some of the submissions to the most recent Decker game jam, including (Don't) Save Me and I Lost My Duck take advantage of this.
There is no grouping mechanism for widgets upon a card other than Contraptions.
Decker's image encodings are described in The Decker File Format. It's also possible to use Lilt to programmatically manipulate decks, encode/decode various resource types and import/export a handful of conventional file types.