Background window colour.

[Onidemon]

I would like this, I'd also like them to keep their promise of providing us tools to change the UI and Interface. I used to do it myself by modding but every patch would force me to put it back to default and redo.

Modds/addons is a great idea.

WOW benifited a lot from them and Blizzard eventually made some of the features, provided by addons, default in the game's UI.