Yes, sorry I did not mean to be cryptic. You can change this in your child theme stylesheet. Here's a very helpful diagram of the divs that make up the pages and posts:
Thematic has a number of stylesheets that are "imported" into your child theme stylesheet - you can look at those files in wp-content/themes/thematic/library/styles. If I am going to change a lot of things in on of those files, I will comment out the @import statement in the child theme stylesheet and paste the contents of that stylesheet directly into my child theme stylesheet.