i'm not sure how exactly he did it, there are multiple ways you could do it. easiest way i could think of would be to create an artframe for the background, place your default chatwindow on top of that and then use a custom autoexec-script to hide the textures and buttons from the default chatwindow.
you can also manage the chatframe and editbox directly in artframes, but that feature does not support docked chat-tabs and combatlog.
its not my ui, i dont know how he did it, the post is over 9 month old, its the only post the author ever made and he states that he does not play wow much, so i would not expact support from him