On the Design tab, in the Themes group, click the More button to open the entire gallery of themes:
Click the 'More' button
Do one of the following:
To apply a built-in theme, under Office or Built-In, click the theme that you want.
To apply a newly-created theme or an existing theme that you previously modified and saved, under Custom, click the theme that you want.
To apply a custom theme or a themed document stored in a different location, click Browse for Themes, and then locate and select the theme you want