Post appearance in Sandbox Theme

I’m trying to style Sandbox, and cannot figure out where to style the appearance of posts. All I want to do is give the posts a white background and a light gray border. I may want to adjust more later, but I can’t seem to find anything that deals with post appearance.

I’ve looked through the forums to try to find an answer but haven’t found one. I’ve figured out most of the styling, and just haven’t finished styling most of my site yet; I’m just hung up on this issue and don’t want to move on until I figure this out.

Thanks for your help WP Community!