[Adam H]

You would just do something like this :

Add this to your additional.css to remove the current top border :

.signature {
    border-top: none;
    margin-top: 2em;
    padding-top: 1em;
}

Then in add this to the additional.css for your new border :

.signatureborder {
    border-top: 1px solid #DDDDDD;
    margin: auto;
    width: 80%;
}

Then goto postbit_legacy and find :

<vb:if condition="$post['signature']">

Directly under it do something like :

<div class="signatureborder"></div>

I havent tested it or anything but you get the general idea .