There is an option to change the background color but it is super buggy and does not work.
For example, I change the background color to white. Instead it turns it into a gradient background that starts with the pink default and ends in white.
Try playing with the feature and you will quickly realize it does not work correctly. You can never set the starting color of a gradient background, or change the color of a solid background.
Also, transparent background (aka no background) color would be nice.
Scheme-it is super useful, but the bugs are really annoying.