In 509.13.16,
The default background color for output controls appears white in Dream Seeker as expected. In the WebClient, the background color for output controls is black for some reason.

Known issue?
Technically I believe what you're seeing is that the output control's background is transparent. I'd have to see the full skin to understand better.
That's what it looks like although not what I would expect since I'm setting the background color to something other than transparent.
If you're setting the control's background color to something besides default ("none" in the raw skin), then yes, it should appear. The only reason it might not is if something is overriding it, like a background color in your .dms styles.