We actually allow any user to save a skin.  But unless you are a Premium member or a Free Trialer, when you save you will see the following message:

Xanga Skins is a Premium Feature
If you'd like to use Xanga Skins on Your Site,
Click Here to Upgrade Today!

If you *are* a Premium member (or Free Trialer), then your Skin will not only be saved but it will also be activated on your site!