When you try to change the theme in the Themes group on the Page Layout tab, Themes is unavailable.
The ability to switch themes has been restricted in the document.
To remove this restriction, use the Manage Styles dialog box.
On the Home tab, in the Styles group, click the Styles Dialog Box Launcher.
In the Styles task pane, click Manage Styles , and then click the Restrict tab.
Under Set availability of selected styles, clear the Block Theme or Scheme switching check box.