Background disappears on single post pages

I am using Atahualpa 3.5.1, WP 2.9.2 (latest version as of 6-15-10)

I designed a site using this theme, and the home page looks good. However, when you expand a post, the background of the page disappears and I cannot figure how to fix it. I checked it in firebug and it all seems to be correct.

Here is the home page http://musingmainiac.com/

Single post page: http://musingmainiac.com/2010/06/sar...-illusion.html

Also how the heck do I make the font weight of the nav bars bold? I have tried font-weight: bold; but it doesn't work. You can't see "home" very well on the home page.