On the Skins & Templates Admin, the Default Skins drop-down lets you to select the skin that each type of user sees by default. Selecting the skin automatically sets it. Affiliates keep the default skin they had when they created an account; only affiliates that sign up after you change the default skin get the new default skin.
You can add a new skin to the drop-down box by creating it and then marking it as active. To activate or deactivate a skin later, click on skin's edit icon and change the setting.
To change the skin for an affiliate in NATS3, log into your administrator account, go to Affiliates Admin, find the affiliate, click the Override icon, go back to Affiliates Admin, and choose a skin from Skins box in the upper-right corner. In NATS4, on the Affiliate My Accounts page, affiliates may choose their own skin.