A lot of people have good design ideas that aren't right for the default theme, e.g. black background. However, I would like to make themes and have those themes available as a setting. So if you'd like to make a theme, here's how to get started.
- Check out the latest unminimized CSS (refresh that link for latest).
- Tweak the CSS to your liking (I like using Firebug).
- Report back your set of CSS changes and the name of your theme.
- I'll check it out and if it looks good add it to the list!