If you're curious, this is what the end result looks like: http://i.imgur.com/NURV2N3.png, and if you use calibre, here's the CSS: https://gist.github.com/osuushi/6461324

You can change the font size and line height by a little bit without needing to adjust anything (since the color gradient is continuous vertically, it is a bit tolerand), but if you make a dramatic change, you'll need to adjust the background height from 72px to the height of three text lines.