Design > Skins

Navigate to the Skins listing screen then select the desired Skin, then scroll down to the bottom of the screen.

On the bottom left hand side you'll find the Action button. Select Copy as the option then click on Action and confirm OK.

The new Skin (the copy) will appear as a draft, with the Detail screen in Edit mode. Set a new Name for the Skin and fill out the obligatory fields/options then Save.