You may want to have a different background on your WordPress website. For example, you may have a celebrity blog and want to include an image of that celebrity on each of there pages or you may have a site that covers different topics and want to have a different color to represent each category.
This is easy to do with the plugin called Background Per Page.
Simply (in your WordPress dashboard) go to Plugins > Add New and search for Background Per Page, or download Background Per Page plugin, extract and upload the folder to the /wp-content/plugins/ directory. Then just Activate the plugin trough ‘Plugins’ menu in WordPress.