Your custom background will appear only on single tweets, some other pages and stop.
The difference is quite clear
“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.”
http://www.wired.com/2015/07/get-twitte ... aper-back/
If you try to update or refresh your custom background, for an instant your TL returns like at the old times. Then, at the 1st refresh, it will resume its new horrible look ...