low and medium background settings on osx result in black background

What Video driver are you using? OpenGL Reference or Open GL 4?
I just tried it on both and Low/Medium settings had backgrounds.
What hardware are you using?

GL 4.

I have a 8-core 4,1-converted-to-5,1 MacPro, ATI Radeon hd 4870, MacOS10.10.5

Try the Reference GL driver just to see if that works, please.