Open up the HTML page in which the gallery is embedded in a plain text editor.
You'll find the line:

var fo = new SWFObject("autoviewer.swf", "autoviewer", "100%", "100%", "8", "#181818");

The last parameter ('#181818') is the background color.  Change this to the desired color, save the document and reupload it (overwriting the original version with the unwanted background color).
See Setting Gallery Size and Background Color on this page for details: http://simpleviewer.net/autoviewer/supp … dding.html