Disclaimer: I'm using my own webhosting service, and my wordpress blog isn't hosted on wordpress.com
There may be another way, but you can update the database. The table is wp_options, and the option_value is "background_color".
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UPDATE _wordpressdb.wp_options SET option_value = ' ' WHERE wp_options.option_id =269;
Another possible option is, login to your admin, then go to Appearance-->Editor. Edit the "Theme Functions" (functions.php) template. Look for the line:
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background-color: <?php if(get_option('background_color') == '') { ?> #<?php echo $color->bg['+2']; } else {<br />
Change the part where it says
get_option('background_color') == ''
to
get_option('background_color') == '#000000'
1. Watch out, those are two single quotes, not one double quote
2. Replace #000000 with any color that you definitely won't use as the background color. Basically, this says, "if the background color is #000000 (black), then use the automatic color"
Hope this helps. And hopefully, the author of this theme can chime in to say if there's a proper way to do it.