If a Punchh email is displaying with a dark or black background in a guest's mobile device but does not appear that way in the email editor in the platform, be sure and check the guest's device for dark mode. Dark mode is a fairly new feature and only available on certain OS's on specific devices. The mode must be enabled.
As a workaround to avoid display issues, it is best practice to build your email template with a user defined background color, to avoid display issues regardless of what mode a device is utilizing.