In most themes you can set background for your website. You can set background globally for the whole website, and also subsequently set a custom one for your separate pages. This chapter is about setting background as a global option, please see Page Background chapter for instructions on how to do this on your separate pages.
To set default background for your website please proceed to Theme Settings > Appearance > Background. here you can set either solid Color background or Image background.
if you want to set an image background, please use a checkbox for this:
Choose an image from your media library or upload a new one:
and the image will appear at the preview: