Older posts not formatted same (no background)

I have a page that holds 5 columns of posts, 5 deep each. I set up the posts to use a yellow background to look like a post-it note (I looked at post-it note-type plugins and am not interested in them). So, when I add a post, and the original is moved down, it does so without the yellow background. I’ve tried to find information without luck.

The page I need help with: [log in to see the link]