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.