Google Sites now supports the ability to apply a site template’s theme to any existing site. This does not apply the entire template to your existing site, just the template's theme.

Editions included:
Standard, Premier, Education, Team, Partner Edition and Google Apps for Government

Languages included:
All languages supported by Google Sites

How to access what's new:
Click 'More actions', then 'Manage site'. Go to 'Themes', then select 'Browse the theme gallery' to choose a template's theme.

For more information:
http://googledocs.blogspot.com/2010/07/apply-template-themes-to-existing-sites.html

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