If you look at the HTML for the header, you'll see something like this;
<th id="head_3">
<div class="nt-left nt-fakeget" data-do="sh">
<a title="Click here to sort by Forward Address" data-value="3">Forward To</a>
</div>
</th>
As you can see the text ("Forward To") is inside an anchor tag (<a>) and the anchir tag is inside a <div> tag.
There's not a simple way to center the div inside the cell, so the only thing you can do is make the div "bigger" and center the text inside the div. That's accomplished by adding ' nt-center nt-width-100' to the Div.
But there's a problem. Other stuff can appear in the header as well - for example the sorting arrow (up or down) or in the case of say Secwin a lock icon in the last column. Other stuff as well is likely to appear here in the future. Setting nt-width-100 causes the div to take the whole width, which in turn makes the icon appear on a separate line, so the header gets ugly.
For "reasonably, but not hugely" large columns something like
' nt-center nt-width-80'
can make it pseudo-centered. But that is somewhat less-convincing when the icon isn't there.
Now it's probably possible, using the position attribute, coupled with left and right settings to do something useful here, but so far I haven't found it yet.
cheers
Bruce