Need to add an image to minimalism theme

Dear all,

This is my first post here so hello to everyone.

I have recently installed wordpress to a site called englishroseweddingdesign.co.uk . The blog is at this address/blog . I used the minimalist theme as it resembled the layout of my site but I really would like to change the white background into the image used on the rest of the site. Unfortunately, I can’t seem to find the css function that relates to this specifically. Can anyone help?

Thank you in advance