chicocrocante on "[Theme: Twenty Eleven] Page background without messing"

My site is http://ianxblog.com.ar
I use the following CSS style:

html #page { background-color: transparent; }
html #site-generator { background-color: transparent; }

I want to make the content part of a post black, but keeping the header and the footer background transparent.

If I set

html #page { background-color: black; }

The header and footer background goes black too.

Thanks