Twitter have removed background images. All gone it seems.
Techcrunch speculate that this is to control the potential advertising real estate and to prevent the service from being too uncontrolled, like Myspace. However, unless I'm misremembering, the full page background image hasn't been around terribly long and having the ability to adjust your background colour but only temporarily is really quite R&D.
"I can go back and change it to orange, but then it goes back to white after a few clicks around the site. Such a weird user experience."
Twitter have a market cap of $23B