If you've been seriously concerned about how your Twitter profile looks, we'll disappoint you. The social network deletes all background images and replaces them with a background in neutral shade of blue.
In the Twitter Design settings, you will see that the image you were using is still there and clicking Save changes looks back.
However, when you visited your profile from another browser that is not linked to your account, you will see that the image no longer exists.
No one knows why Twitter has decided to remove the profile customization feature.
Perhaps because he wants to apply a uniform style, just like Facebook does, the background is the same for everyone.