There are some theme options that are part of the WordPress core and those options need to be accessed through the wp-admin. One such option is the WordPress Background Editor. It offers the ability to set a background color for your site, or to add a background image.
Go to the Appearance panel in your wp-admin and click on the Background link to access the Custom Background page. To set a color, you can use a color picker or type in a hex code value. To use an image, you must first upload it using the browse and upload controls. Then you need to select how you would like it to be positioned, whether you want it to repeat or if you want it to be attached. Each option is easily selected with the radio buttons.