No matter what value I enter in the Primary Menu Bar > Desktop Menu Spacing box, no space between menubar items is added. What happens, is that all of the menubar items move to the left, so that they are no longer centered (but with the same space between them). See it here: http://www.nordoy.net/ In this case I have set the value in the Menu Spacing Box to 5 (em). With a value of zero it’s perfectly centered, and with a value of e.g. 10 the menubar items move even farther to the left.
I have with tried different values, with no result.