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If you're talking about the custom block (center one) you have on the main page, just go to the theme_blockcenter_c.html and lose the padding and <fieldset> tags. (back up the file first!) reload and see if thats the look you was going for. if you want more, you'll have to go into the style.css and edit it to remove the borders, etc.
Make sure to click the update themes in the general settings to YES or it wont show. Once is shows, you can set it back to NO.