It will probably be sufficient to add the same background color to the divs Plogger uses, like the wrapper etc. Look inside the /css/gallery.css for the correct names and add background:url(imagedirectory/imagename.gif); to that div.
If you just want a color, use this in gallery.css:
body { background: #000000; }
You can change it to any of 16,000+ colors by using the hex value. Find the hex value of any pixel on your screen with this little gem: http://www.nattyware.com/pixie.html.
... of course that would be sufficient, but I was counting on the fact that his plogger is integrated into his site, and then it would already have a backgroundcolor...