* For GtkWindows, set the border-width to 12, unless it needs a
  menu, in which case the window should have 0 border-width,
  and an inner GtkBox, a sub-GtkBox and GtkButtonBox should have
  a border width of 6, allowing the menu to fit snugly into the window edge.
  Using just a border-width of 12 on both the GtkBox and the
  GtkButtonBox would leave 24 pixels between them.
* For GtkDialog, set the border-width to 6 and give the GtkBox in
  the content area another border-width of 6, because the
  GtkButtonBox in the action area seems to have this already too.

  I do not know why GtkDialogs do not just have a border-width of
  12, and a VBox spacing of 12.