https://help.libreoffice.org/Common/Defining_Background_Colors_or_Background_Graphics says: 1. Choose Format - Page. 2. On the Background tab page, select a background color or a background graphic. In spreadsheets this background appears only in the print behind the cells not formatted elsewhere. We don't have Background page, we have Area page. Please review and fix related help pages.
*** This bug has been marked as a duplicate of bug 96496 ***