I had to look it up, but it sounds farily similar to the way wordpress works. And actually, many gpEasy themes are already distributed with multiple variations ( what wordpress would call "children" ).
For example, looking at the default theme "One Point 5" in your installation you'll see multiple variations. Open up the folder /your-install-folder/themes/One_Point_5 and you'll see multiple folders that represent each varation. "Blue-green" is essentially a child of "Blue". So to create a new child, you could just copy the "Blue-green" folder and edit the copied files.
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