I think it looks nice, very sleek and minimalistic which is a style that I like. I had no idea that CSS could do drop shadows. That's pretty neat. One suggestion: calibre should be spelled with a lowercase C as that is the official spelling.
Don't worry about stepping on toes. We can't improve if we aren't willing to risk exploring new options.
And please don't remove the link from the post. It's annoying when people go back and self-destruct a thread for no good reason. Some people may not read this post and respond for several weeks (on vacation, busy, etc). Or someone may come along in a month or two and have some great input about it.
BTW, it's nice to finally see what calibre looks like on the Mac! I've heard Mac people complain about the look, but it looks much nicer in your screenshot than it does on my Windows computer.