One small addition: in case of 'overwriting' an element that has properties, thos properties have to be in the dependant skin too, even if they don't change.
For example: imagine you want to 'overwrite' this element in the dependant skin:
<element id="Content[Artwork]" bg="X,X,X" fg="X,X,X" bdr="X,X,X" />
That case you should include those elements in the depedant skin too, even if they share the same values in the dependant and the original skin:
<element id="Content[Artwork].HeaderControl.Default" bg="X,X,X" fg="X,X,X" bdr="X,X,X"/>
<element id="Content[Artwork].Body.Default" bg="X,X,X" fg="X,X,X" bdr="X,X,X" />
Until I realized that it gave me some problems!!!