Ah that…OK, I have some bad news and some good news for you. When I made this theme, I didn’t think about the fact that changing the content background colour also requires the curve to change so it matches the same colour. The problem is that the curve is an image, so using CSS is not possible; the bad news you can’t change it.
The good news is that I have the graphic for it, so I can change the colour of it and send it to you. What is the colour of your page background?