How to change BG color of where posts sit
 

I'm not exactly sure of the termino9logy so sorry for that title. My question is in what template can I change the color of the background where posts sit? Current it uses "alt1" . I w want to maintain where alt1 is used in the other sections of my forum cause i like how the color works there but for where the posts are displayed* I'd rather be using "alt2" . If anyone can help me with what template and where to change this code* that would be awesome.

Thanks