If you were using Twitter's default background you many notice that it has change. Twitter has new default background and avatar. You can change it back by clicking on "settings" (top right), "design" tab, then clicking on which ever your choice.
"If you like using Twitter.com as your Twitter client, then you might be interested to know that we've added four more themes to the design collection. We've also updated the default theme and avatar to match our front page. If you prefer the classic default theme, it's still there and you can change it back."