Column header vertical bar

How do I change the color of the small vertical bar separating column names, on the header? I don't think that is a border...
I can't find a way to modify that on grid.css...
Please, help!!
Thanks..

inside the file AW/runtime/styles/xp/grid.png

yes, the vertical bar implemented as an image (part of grid.png):

.active-box-resize {
    position: absolute;
    overflow: hidden;
    top: 15%;
    right: -5px;
    width: 10px;
    height: 70%;
    font-size: 100px;
    background: url(grid.png) -75px 0px no-repeat;
    cursor: e-resize;
}