Change the background color or image of my site and posts

I don’t know my terms well yet…What wordpress calls the “background” isn’t what I’m trying to change. Basically, the “paper” I’m writing my post on is completely white. My main page has a nice image I uploaded, but I don’t know how to get this or any other image, or any simple color as the paper surface for my posts. They’re always completely white.

I see the very back-background is black, and I can change a secondary background color there too. Right now I’ve got black on the top half of the page with dark grey under. But the foreground is white.

I’d love to be able to add an image or color. Not a fan of stark white.

I know this is pretty basic but my initial search didn’t bring up this exact topic and nothing I can find to change in dashboard touches this white screen.

Thanks!

The page I need help with: [log in to see the link]