You want to apply a different theme to a document, but the Themes option is unavailable.
The ability to switch themes has been restricted in the document.
Use the Manage Styles dialog box to remove this restriction.
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.