I have uploaded a new custom background with backgroundmanager in the moonbeam theme.
It doesn't show up on the regular pages but it DOES showup in the search and error page. Of course I also ticked the checkboxes for 'front page', 'single page', etc.
http://mtimmerfotografie.nl (no custom background)
http://mtimmerfotografie.nl/doesnotexist (DOES show custom background)
Any clues?