View through whatever camera you wish to render with...
In the viewport frame (camera1, persp, or whatever you use to render)
Camera Attribute Editor ->
Environment (you'll need to scroll down to see THAT one) ->
Edit the background color...
Considering you said you wanted white you could just get away with moving the slider... but if you double click on the color you can change it to whatever you wish... just like any other windows color pickers like in Photoshop or Paint.