Hey,
The borders on the side are set in #content_section in the css, from frame_repeat.png, so just remove that. As for the top (and bottom) one that's part of the main_block.png. You *could* change the background positions on #header, #header div.frame, #footer_section and #footer_section div.frame to remove the top/bottom pixel, but it's better to edit the image in Photoshop (or the free alternative GIMP) to fix it "properly".