Make sure that you have no background image set via Appearance > Backgrounds. Then use the "BACKGROUND OPTIONS" inside the RT-Theme 15 Options > General Options to set a background image as the demo site. For more help please visit our support forum at http://support-rt.com
Please allow me to share with all the professionalism and wealth of knowledge Neo of RT-Themes has.
I was having major trouble upgrading my beautiful RT-Theme15 theme, and Neo took great care and patience in not only fixing the problem, which was created by my hosting company, but he took the time to get my computer better organized. He did all of this with such a personable manor, though we were working virtually.
I can say that I will truly be a repeat customer and will encourage others to be as well. Excellent, knowledgeable customer service is hard to come by these days, and Neo has demonstrated that they are truly the flagship for it.
Kudos for the beautifully designed theme and stellar support!