I don't think there is one, but its pretty straight forward. Go to the toolbar icon section that you want to change and import the default. then choose to edit it. hopefully you have it set to export as png because this makes editting easier. the set of default icons is like a guide to tell you what icon goes where without having to go back and forth between the editter and skinstudio (you could print off the list). now you can start making your own icons either on top of the default ones in a new layer, or in another image of the same size as the default. once you have made the icons for it. put the whole of the set back into the default png that you chose to edit from skinstudio. get rid of the default icons and save making sure that all of your new icons are lined up correctly. then reload the skin and you should be set.
If anyone can think of a better method, I'm all ears.