For Aviator template, how do you add a thick border (via CSS and/or graphic) that separates blog posts and that spans the full width of the container?

This could really be a help for many users and hopefully the solution works across many templates. Trying to give some visual separation between blog posts and want to have a way that automatically adds a border graphic and the end of a blog post.

If it helps, here's a screen capture of what I'm trying to do.

http://screencast.com/t/tDtTVyyv