Oh okay, then I got it the first time. 
You can apply the background to body as I showed you above, then if you want a transparent background...
You have to set it for #wrapper to begin with.
Then I'm not sure which one of the following: .forum_category,
.forumposts,
.content_category
And some of the following as well:
.board_row,
.childboard_row,
.forumposts > div,
.forumposts > li,
.content_category > div,
.content_category > li
And you may (or may not, depends) add some transparency here as well:
#top_section
But I'm not really good with CSS...