Change the Skin of a User
Steps
A user can be given a skin of their own, used for every page that user opens. Skin Levels explains where that setting sits among the others. The skin is a property of the user's profile page, set through the Objects editor. You need edit right on that profile, and your own user type has to be Advanced: the "Objects" entry of the "Edit" menu is shown to advanced users only, and yours is set in your profile under "Preferences", in the "Editor Preferences" section.
- Go to the user's profile page, open the menu next to the "Edit" button and select "Objects":

- Expand the object of type XWiki.XWikiUsers, then set its "Skin" property to one of the skins the wiki can use. The field suggests pages of the wiki rather than skins, so type the skin's name and pick the "Select ..." entry it offers:

- Click "Save & View".
- Reopen the Objects editor and check that the "Skin" property holds the skin you chose. Every page this user opens is displayed with it from now on:

FAQ
Why is there no "Skin" field in the profile preferences?
The profile sheet does not display the skin property. "Preferences" shows the display, accessibility, localization, editor and extension settings only, so the Objects editor is the one place the property is exposed. That is why changing a user's skin is an administrator's procedure and not something the user does for themselves.
Whose skin wins if the wiki and the user both set one?
The user's. A skin set on a profile wins over the one set on the space and on the wiki; Skin Levels gives the full order.