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Most often, an application may just need to override a template (see Template Loading) so that a certain control renders differently. Or, an application may need to add a new template to an existing theme. Other times, you might want to create an entirely new theme, perhaps because you are building a rich set of unique and reusable templates for your organization.

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The xhtml theme provides several good examples of the "wrapping" technique. The simple theme renders the basic control. The xhtml theme "dresses up" many of the controls by adding a header and footer.

<#include "/${parameters.templateDir}/${parameters.expandTheme}/controlheader.ftl" />
<#include "/${parameters.templateDir}/simple/xxx.ftl" />
<#include "/${parameters.templateDir}/${parameters.expandTheme}/controlfooter.ftl" />

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parent = xhtml

An extended theme does not need to implement every single template that the Struts Tags expect. It only needs to implement the templates that change. The other templates are loaded from the parent template.

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Using expandTheme parameter allows to override only some parts of the theme's templates, e.g. css.ftl. You can define a new theme (set theme.properties) and override just single file.

${parameters.expandTheme} is a recurrence which tells ThemeManager to load template from current theme and then from parent theme (defined in theme.properties) and so on.

Please also notice that the ThemeManager builds list of possible templates based on current theme and inherited themes (/template/custom/textarea.ftl, /template/xhtml/textarea.ftl, /template/simple/textarea.ftl). This is also true for templates which are loaded via ${parameters.expandTheme}.

See also example Struts 2 Themes or Creating a Theme in Struts 2 (Mark Menard)