Hello,
I have look at some of the theme listed. I am afraid we can't do anything much, as it is that way by design. The theme author have put a gap between the top of the visible topframe and the actual top. Reason for putting there is depend on design. Usually for back shadows for the windows frame.
Here s/shots of some top frame taken from actual theme.
1) Diamond : 2 pixels gap.

2) Corporate and Ice : has special Shadow frame added.

So when you push the topframe to the edge of your screen. That extra pixel will always be there but you can't really see it.
Thank you,
Basj,
Stardock Community Assistant