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
In an unannounced move, Twitter no longer lets you see the customized background you added in the design area of your settings on the home or notification screen pages. Womp Womp. We asked Twitter “wuz up wit dat?” and a spokesperson said: We’re removing background images from the home and notifications timelines on web for all users. Now, background images are only available where logged-in users will see them publicly (Tweet pages, list pages and collections pages). You can find help center content about customizing your design and where it’s visible on Twitter here.