This is one of the caveats of the curve for this theme, you cannot change the colour of it because it’s an image. You would have to create another curved graphic or take the existing one and colour it.
One of the things I am hoping to do at some point is to make that curve pure CSS so that it can be easily colourized from theme settings.
As for changing the background colour of a single page….you can, but you need to target that page with your own custom CSS using one of the classes or ID’s of the tag in the page source code and then do your background colour for the page in question. You would have to apply the background colour to the container that does the content area. You also need to do a forced background colour like this example:
background-color: #cccccc !important;
The !important part of that code forces the styling to use that.