Each theme has its own styles therefore often the background color is fixed within the styles.css file however you can still overwrite this by setting a new one yourself in the main style.css file.
Todo this login into your WordPress admin as the admin/super admin, under the left menu “appearance” click “Editor” then locate the file “style.css”.
Add/edit the body tag like this;
body {background-color: green !important;}
if you would like to set a background image the code is;
background-image:url('http://www.mywebsite.com/link-to-my-image.gif');
