"It’s probably worth pointing out that though this tool was developed to make creating themes for SharePoint easier, it works with any website and has no real dependencies on SharePoint.

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As noted in the discussion of the image palette, all of these edits are actually taking place locally and have no effect on the server.

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Select Export Theme from the File Menu. Give your theme a name that will be used on the SharePoint server, pick a directory, and viola! You have a theme. Now all you have to do is install it."

SharePoint Skinner Overview