Assign 
a Theme to a Portal Element 
Themes allow you to customize specific portal elements 
  as books, pages, or portlets. As an administrator, you can select pre-existing 
  themes and assign them to elements. There is no limit to the number of times 
  a theme can be used.
To assign a theme to a portal element:
  - In the Portal Resources tree, select a book, page, 
    or portlet whose theme you want to change.
- Select the <portal element> Properties 
    tab in the Editor pane if it is not already selected. For example: Book Properties. 
  
- In the Theme pull-down menu, select a theme.
- Click Save.
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