Background Color, Background Image, And Header Image Setup
Please go to Appearance >> Customizer >> you can see Color, Header Image Section, and Background Image Section. To change background color select Color Section and set your background color for your website.
Go to Header Image Section to change the header image, and upload header image here for your website. And to change the background image on your site, Go to Background Image Section and upload your background image.
After that, You can click to save these settings on Top Publish Button.