Its not a defacement. Looks like you block our ads so you've probably never seen our skin ads before. The hint is the "Advertisement" text in the upper left.
A skin ad replaces the background image and its how we pay for the terabytes of bandwidth we are flying threw this week. Also will allow us to get more servers for future patches.
Its a good campaign to run being close to Halloween. I know I like my scary movies around this time.