[Twenty Eleven] Background behind featured post

Hello there,

I have created a child theme of Twenty Eleven. I use the showcase template when you first visit my blog and I have made a blog page to show all blog posts together.
In this blog page (not in showcase) I want to put a background behind featured “sticky” posts (as Twenty Ten did), but I can’t find the right element in css to do so. How can I do that?

My blog is here: https://blog.elemedu.gr/blog/

I would appreciate your help.

Thank you.