Here you learn how to apply a new theme to a SharePoint site and then review subsequent changes both within the browser and within SharePoint Designer 2010.
To change a site’s current theme, complete the following steps.
You need a permission level of design (or be a member of the out-of-the-box SharePoint Designers group) in order to apply a new theme to a SharePoint site.
1. Open the root site of your site collection in your browser and navigate to the Home page of the site.
2. Alternatively, if you already have the equivalent site open in SharePoint Designer 2010, from under the Navigation pane select the home page of your site and then in the right-hand side of the screen, under Customization, click ...