Well, backgrounds are just your wallpaper. I highly recommend Wallpaper Wizardrii for resizing pics. Just install it, put whatever pic you want on your card, long press on a blank spot on a homepage, choose wallpaper, then choose Wizardrii.
Some themes have custom icons, check out the sticky guide for those. I'm running Liberty with the Poison Green theme, and it is green.
As far as the boot screen goes, I haven't messed around with it yet. I was planning on hunting around and gathering info on it for the guide in the next day or two.
As far as creating your own themes and icons and whatnot, mostly UOT Kitchen is used by the devs. I'm not familiar with it yet, but I'm planning to be fairly soon.