To add background color or image to a site which will be applied to all pages, you need do the following:
1. From the WordPress left menu, go to Appearance > Customize.
2. From the Customizer left dashboard, go to Site Settings.
3. In the Site Settings popup, go to Styles tab > Main.
4. Add your custom background color or image.
5. Close the popup and Publish the changes.
Setting background for custom devices
Sometimes you need to set backgrounds differently on different devices. In Customizer, there is a tool which lets you do it. But it’s available only for the places where it needs different backgrounds.