Apparently it's just an experiment,but if you want to turn it off, there's a Plain White theme you can choose.
lifehacker.com/5559961/turn-off-googlecoms-24+hour-background-image
(lifehacker.com/5559961/turn-off-googlecoms-24+hour-background-image
Kamikaze Stoat £4.09,
Thu 10 Jun 2010, 14:51,
archived)