What theme? I’m guessing Kubrick? (The standard wordpress theme.) If that’s the case, that whole background is an image. Log in, then go to Presentations > Theme Editor. Click on “Header.php” and look for the following code:
body { background: url("<?php bloginfo('stylesheet_directory'); ?>/images/kubrickbgcolor.jpg"); }
<?php /* Checks to see whether it needs a sidebar or not */ if ((! $withcomments) && (! is_single())) { ?>
#page { background: url("<?php bloginfo('stylesheet_directory'); ?>/images/kubrickbg.jpg") repeat-y top; border: none; }
<?php } else { // No sidebar ?>
#page { background: url("<?php bloginfo('stylesheet_directory'); ?>/images/kubrickbgwide.jpg") repeat-y top; border: none; }
<?php } ?>
#header { background: url("<?php bloginfo('stylesheet_directory'); ?>/images/kubrickheader.jpg") no-repeat bottom center; }
#footer { background: url("<?php bloginfo('stylesheet_directory'); ?>/images/kubrickfooter.jpg") no-repeat bottom; border: none;}
What you’re looking for is this line:
#page { background: url("<?php bloginfo('stylesheet_directory'); ?>/images/kubrickbg.jpg") repeat-y top; border: none; }
<?php } else { // No sidebar ?>
Comment out that reference and you should be set to go.
Note that the other images intertwine rather nicely with it so if you make a change to that — say, by commenting out that line — it may look odd on the page.
Any more questions I’ll be glad to help with, insofar as I can. (I did the same thing on my site, eliminating the gray bar.)