You need to edit the style.php file to remove the gradient from the background.  Just look for this code:

#bgwrapper {  /*— This id selector controls the blog background layer —*/
    width: 100%;
    margin: 0px;
    padding-top:<?php echo $flex_blog_margin; ?>px;
    background: url(images/bg-fade.png) repeat-x top left;
    min-height: 100%;
}

and change it to:

#bgwrapper {  /*— This id selector controls the blog background layer —*/
    width: 100%;
    margin: 0px;
    padding-top:<?php echo $flex_blog_margin; ?>px;
    min-height: 100%;
}