Background style

I have a theme that i have amended.
I have a content div that houses the following divs:

L_sidebar
contentleft
r_sidebar

The content div has a background styled into it that grows with the div as the other 3 divs force its height to grow.

this works great until i click on the comments link then the background disappears.
I have checked the code with firebug and the style is still there but the div is no longer growing so you can’t see the background.

any ideas would be great.

here is a link to the site i am working on.

here