A question that often comes up is how to adjust the background color for demos that are created using the Flat theme. Right now you have the option between a dark mode or the lighter version in gray.
This is actually a rather simple process. As you start to input your color scheme for the demo when you get to Title Text Color, using a dark text color will have a lighter background. However, If you were to change this text color to something lighter you will notice that the background color changes to black.
As you are building out your demo, reference the preview image in the top right corner to see which text colors will have a light or dark background. Below you can see the difference between the two.