Using thicker border for focus in GTK3 theme

Question

I'm trying to make a gtk3 theme where the focused control is highlighted with a thicker border:

GtkButton /* etc. */ { 
  border-width: 1px;
}

.focused {
  border-width: 2px;
}

This works, but is ugly as focusing a control increases its size, moving nearby controls around a tiny bit. (This is clearly visible in twf and friends.)

Is there some simple CSS solution to increase a border without changing the overall dimension of the control?

My system is ubuntu 12.04 (precise) with Gnome 3.4.1 and I'm using the unico theme engine.