Yes, by using the <ww:set name="theme" value="myTheme" scope="page" />.

This will set the attribute 'theme' in the page scope, with the value of what that is returned by 'myTheme' in the stack, eg. the action class might have a getMyTheme method that return a String called simple, to indicate simple theme.

Another way would be <ww:set name="theme" value="'simple'" scope="page" />

This will set the attribute 'theme' in the page scope with the value 'simple' to indicate a simple theme.

See Selecting Themes