Pretty simple - choose your background and text color for Sweepstakes and Boomtime Deals.
Because they are rendered in social media and in widgets, the colors you can pick are kept to a minimum, and default to black and white. Don't worry about the layout of the Preview - it is more to let you see how the text looks on the background color.
When the Sweepstakes or Deal is running, it will be contained in its own little box so the "floating in a sea of white" should scare you too much. :-)