[webmastersun]

your idea looks like my forum border that I made http://www.webmastersun.com ?
you need create a image file with height 1px and width depends on your forum width

add css

.mainwrapper
{
  width: 1025px;
  margin-top: 0px;
  margin-right: auto;
  margin-bottom: 0px;
  margin-left: auto;
  background-color: transparent;
  background-image: url("http://www.webmastersun.com/images/piewrapm.png");
  background-repeat: repeat-y;
  background-attachment: scroll;
  background-position: left top;
  background-clip: border-box;
  background-origin: padding-box;
  background-size: auto auto;
  z-index: -1;
}

If you have problem let me know.

Good luck!