1. Global Background Setting
Via the Customizer you can quickly and easily change your background. Simply go to Appearance / Customize / General Options and here you will be able to locate the “Site Background” options. From here on it should be very straight forward. Have a look at the screenshot below:
2. Per-Page or Per-Post Backgrounds
If you want to alter the main background on a per-post or per-page basis you can do so as well via the built-in meta options (custom fields). Simply go to the page or post you wish to edit and below the editor (or somewhere on the page depending if you moved your modules around or not) you will see the page settings metabox where you can click on the “Background” tab to easily alter your background.