Apologies for the self-promotional nature of this topic, but I think it is particularly relevant and helpful:
Recently I’ve been taking advantage of the WordPress.com custom design upgrade to pimp my theme, so I thought I would share some of the CSS I have written. Twenty Eleven is the second most popular theme in the WP.com directory, so why not make yours stand out too?
If you use Twenty Eleven you can just help yourself to the CSS here. Even if you don't, the code may still be helpful. I also talk about the custom design upgrade and the CSS editor.
I'll be happy to help anyone with questions either here or in the comments on my post.
The blog I need help with is halfblog.net.