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?
Contact me and I will email you the new curve graphic. Please be aware whenever there is an update to this theme, you will have to keep replacing the curved graphic, otherwise, if you are using a child theme, then you can upload this image to your child theme images folder and then copy over and edit the header.php file. But email me directly and I will send you the png image. If you have Photoshop, I can send you the PSD file so you can make different curves with different colours if you change it later. Do you have Photoshop?