I thought I'd tried it, since I know you can add book cover images that way. I thought it hadn't worked, which is why I was asking. But maybe I just need to drink less rum. (Or more!)
Oh wait ... so how do I make a template CSS in the first place? I can't export one out of Sigil (that was the other thing I was going to ask!). Is a CSS file just a text file renamed? Do i just copy/paste into notepad and save it?