UIElement: Change Game Screen Background

Hi All,
I have a requirement to keep a constant aspect ratio regardless of screen dimensions (see screenshot). I do this by changing the camera Viewport Rect based on screen dimensions. This works fine and I've used it before but I need the Game view background to be black, at the moment its a gradient from grey to blue.
I'm sure I've changed this easily in the past but can't seem to find where, and I notice in the UIElements Debugger it's all part of the new system I wondered if it's changed since I last changed it.
Does anyone know if I can change the background?