To change the background of the viewer to a color, add the following style into HeadCSS with your desired hex color:
body
{
background-color:#000000;
}
If you do change the background of the viewer, keep in mind you may need to change the keyline border around the publication, as well as the toolbar spritemap.
Please refer to the links below.
Keyline:
http://support.realviewdigital.com/solution/categories/40851/folders/65958/articles/72739-viewport-border
Spritemap:
http://support.realviewdigital.com/solution/categories/40851/folders/65958/articles/30858-changing-the-toolbar