Background image for the entire page

Summary:

simplify differentiating between the TEST instance and PROD instance by adding a background image on the page

Content (required):

when I scroll down the item page, the top banner (with color and image to indicate test instance) disappears. Now, user does not know whether the record is from the TEST instance or PROD instance. User has to check the URL or scroll back up. I am trying to simplify the difference by adding a background image on the page. However, there does not seem to be an option for this. Does anyone know of a way? I tried to edit the global page template and edit page options.