To get a background image in Calibre, I gather that I need to open a book and right click. This will expose a menu. Choosing "Preferences" >> "Styles" opens a GUI to change the background along with a place for more detailed editing of the CSS. The GUI ends up leaving a white border around the background, which I found I can remove by going to "Page Layout".
Any help would be appreciated. This is calibre 5.6.0.