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#use this to get the skins directory
ps = getToolByName(self,'portal_skins')

# use this to add a new folder to that directory for your new skin
ps.manage_addFolder(skin_id, skin_id)

newSkinFolder = getattr(ps, skin_id)
path = newSkinFolder.absolute_url()

# get a set of tuples of all skins
paths = ps.getSkinPaths()

# these lines get the layers from Plone Default
defPath = paths[0]
defPath = defPath[1]

# add your new folder to the top of the Plone Default layers
defPath = skin_id "," defPath

# add the skin to the properties of portal_skins
ps.addSkinSelection(skin_id, defPath, 0, 0)