Struts2 provides four different themes: simple, xhtml, css_xhmtl and ajax.

In all my projects I use the simple theme which behaves exactly the way we would expect regular html to behave, since we are all web developers and know html the simple theme provides the least astonishment (and does what we expect everytime).  Because I want this theme to apply to my whole application I simply set this value in my struts.xml.  The default theme can be set for a specific tag, the page being rendered, the whole request, session or even globally.  For more detail on themes visit struts.apache.org.

Struts.xml - globally setting the theme to simple

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE struts PUBLIC
    "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
    "http://struts.apache.org/dtds/struts-2.0.dtd">

<struts>
    <constant name="struts.ui.theme" value="simple" />
</struts>