Moodle 3.6 Fordson v3.6 release 1.3.1 - Theme preset: Perception
I went to: Site administration > Appearance > Themes > Fordson > Custom image settings
Then I went to "Default page background image" and uploaded a new image.
I saved the changes, but it still uses the old image.
I think I figured it out. I thought the image of the shadow of a guy with a graduation cap was the background, but I guess that was the header.
I changed the "Default header image " and it seemed to work.
Fordson uses the header image so that teachers can customize the images within their own courses.