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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)