[SgtTechHead]

In side_borders you are setting a width of 762 and declaring auto margins left and right:

/* ***** side borders ***** */ 
.side_borders {
width: 762px; 
border-left: 2px solid #0e0e0f;
border-right: 2px solid #0e0e0f;
margin-left: auto;
margin-right: auto;
}

Yet, your forum width is set at 900. The side_borders is overriding the margins using 762 for a width. Make the width in side_borders 900 too and it should correct itself.