Thanks, I hope you have fun playing with Decker!
It's possible to have 16-color backgrounds in Decker right now, but kind of a pain: the "read[]" function can import color images posterized to the 16-color Macintosh palette. If you prepared a set of appropriately dithered and colored images with some external tool, you could use a Lilt script to bulk import the images and splat them into card backgrounds. I've added a more direct/integrated workflow for color images to my todo list.
256 colors would be trickier. Decker/Lil Images are 256 color internally, but some of those are reserved for 1-bit patterns, animated patterns, and special UI-related "magic colors". With some light hacking on the codebase you could easily reclaim at least 128 more usable colors.
Deeper audio bit depth is another simple tweak from a code perspective, but it'll make deck filesizes get huge very fast, even if I add better compression methods. I don't think I'll be going that route in the forseeable future. Perhaps something more like .MOD playback eventually would be a better fit.