Should be able to change it. Not at my Mac right now, but if no one else can help you I’ll take a look later today. An URL might help someone help you.
The background color is controlled by the theme, not the plug-in. You didn’t say what theme you are using, but unless the theme developer chooses to change or make this a setting, your going to be stuck with this color.
The dark background is just the photo album one and not the background of the page. The theme is Simple. The color is Grey.
The photo album background color flash plugin in RW 5 was white. But now it’s not. I think the control of the color is inside the plugin code made by Monoslideshow. So I hope RW developpers will ask Monoslideshow owner to make some changes.