The default design has a white background, so if you design the landing/registration pages with dark backgrounds, you've got an ugly white box in the middle of the lovely page.
Thank you for bringing your query to the Community forum. Unfortunately, there isn't a way to edit the box that comes up when they register. I can definitely get your feedback to the proper channels. I can see how being able to edit that box would be a great feature! Thank you for your feedback, and I'm very sorry there isn't a way, currently, to edit that box. Let us know if you have any other Event registration questions!
@gwendolyn_B, I do hope that you bring this up through the specified channels, and I also hope you can provide me with the response you receive.
I have set you up to be notified when a resolution happens! Please let us know if we can assist you with anything else!