ThemeBuilder is a Theme in the sense that it can be selected as an alternative to the default Confluence theme in Confluence Administration > Look and Feel
A skin is the collection of editable panels that make up one whole page framework that can be applied to Confluence through ThemeBuilder. You can have multiple skins in the Skin Editor, and each can be applied automatically to individual spaces, pages, actions and similar options, to finely tune the user experience.
In previous versions of Theme Builder a skin was called the layout.
Icon menu for actions you can apply to skins
(from left to right)
Tree navigation showing all the skins available
Click the headings on the left for more information on that item.
Add and edit the skin name and description.
Editor view for theme code, with context colour coding.
Editor for custom CSS, with options for IE only and CSS includes.
Advanced skin, site and SEO options.
Customise the site DOCTYPE, add HTML to the Header, Body and Footer elements.
Set permissions by Confluence group for skin selection and editing, and panel editing.
Add a text or image resource to be used by ThemeBuilder in your skin.