Here's an "iBook" theme I created for my 13" Macbook Pro (1280 x 800) display:
Should be able to just cut 'n' paste this -- it has a base64 encoded png of a "book" as the image-background. Works well in full screen mode on my 1280x800 display -- probably won't work on other display sizes. Thanks to the base64 encoding there's a lot of css text, so it may take a minute to load into your "user stylesheet" tab (be prepared for spinning rainbow wheel).
Mac users: if you want to craft your own, use following css along with the background image of your choice (put the image in /Users/<your username>/Library/Preferences/calibre/images):
h1, h2, h3, h4, h5, h6