Placing an image as a background - Berlin Theme

I'm trying to place an image as a background for our site (using the Berlin theme), however, I'm not quite sure how to do it. Can anyone help me out or lead me to someone that can?

If you're just editing the theme, you want to add your image to the theme somewhere (like the images folder) and then reference that image in the theme's css/style.css.

There's nothing really specific to Omeka in how you'd go about doing it; just regular CSS.