In many themes you can add a custom background to any page, like a color background or image. You can as well set it be default, just like on the whole site.
To set a custom backgroung for the page you need to unmark the checkbox for the default background, and then you will be able to set background color or image to your preference. To enable background image mark "Show" for Background Image Visibility:
You may choose a background image from your media library or upload it from your computer, just make sure the file size does not exceed 2MB