You can't. Here's a link previous posts:
Thanks for the kudos. I think I must be the only person in Australia who likes the dark background, I find it more relaxing than the bright white background. I also set my mobile phone to have to have a black background so perhaps that's the reason I don't find it so strange.
Let's hope Optus pass on all the comment s to Fetch and they add an option for the background colour.