vB4 Sepecial background for only certain forums
 

I search around and could not find a similar question. I have six forums on my site. Two are "private" and four are open. Some member have access to all six, others have only access to the four open forums.

I would like to make the background of all threads in the private forums a different color than the open forums, that way it reminds users if they are in an open or private forum when they are reading or responding to a thread.

I tried making a child style and changing the background colors. Surprisingly this changes the background of all forums despite how I associate styles and forums.

Can anyone advise me how best to accomplish what I am trying to do? I am not a CSS expert.

Thanks.

Paul