Now I'm seeing what you described (in Firefox), after refreshing the screen again. Both "tabs" are full width, Shop Our Sale above New Arrivals, with the third bar below them. That third bar is the stock centerbox heading background:
Code:
.leftBoxHeading, .centerBoxHeading, .rightBoxHeading {
    background: url("../images/sidebox-bk-1.png") repeat-x scroll 0 0 transparent;
    color: #FFFFFD;
    font-size: 12px;
    font-weight: bold;
    height: 50px;
    margin: 0;
    padding: 15px 0.2em 0.5em;
}
The first two are the tab backgrounds:
Code:
.moduleBox {
    background: url("../images/sidebox-bk-1.png") repeat-x scroll 0 0 transparent;
    height: 50px;
    margin-top: -29px;
    padding-left: 5px;
    width: 637px;
}
You really just need to post in the template support thread, as Anne is the one who knows how this is supposed to work. I don't have experience with javascript.