Twitter has removed the customized background the users added in the design area of their settings on the home or notification screen pages.
According to the Techcrunch.com, a Twitter spokesperson said that they are removing background images from the home and notifications timelines on web for all users.
He also said that now background images are only available where logged-in users will see them publicly (Tweet pages, list pages and collections pages).
The images have been replaced with a blank background with a very slight hint of blue.
The reason is not clear why Twitter would limit users' ability to customize their profiles this way.