How to get a white background/body over print background in coraline theme

Hello WordPress users,

As you can see on my blog, https://fashionatzalando.wordpress.com/, I am having problems with the layout of the white body that I am making on top of my print background.

I would like the white background to be much wider so that it includes all text, I am also wondering if it is possible to make the height bigger too, so that the header will be included?

The header (in addition with all the text and widgets) looks best on white.

I am using the coraline theme, editing the design in the css area and I am not sure what wordpress version I have. I signed up for an account (did not download anything I think) about a week ago.

I would be so thankful for any answers or simplified coding you could give me to make my design look better!