You may also set the theme at the <s:form> tag level, or on individual form elements, like <s:textfield>. IMO setting a scoped variable is the least-readable way of doing it.
From the page I linked to there's a page called
Selecting Themes. It has a list of the mechanisms used for selecting themes. If you're always using the same theme set the property, otherwise set it on the form or form element.