OK, I've tried but haven't been able to create the existing Xcillion skin with a new name.
I can't see my new skin listed in the Themes section under Host > Extensions. The 'skin' was created by simply copying the Xcillion skin and container directories.
So as it's not listed, I tried 'create package' for the Xcillion skin. I can't see how to rename it. From the list of skins/themes I clicked the edit button for Xcillion, that displays a popup form with the 'create package' button which I also clicked. I then clicked 'Next' to display the list of files to be included and the source directory. I clicked next again to display the manifest (which still contains references to Xcillion obviously), then next again where I could change the name of the manifest and the name of the zip file. I can't see how I can change the name of the Theme anywhere.
I have the feeling I'm missing something rather silly.