You should look at the HTML source after rendering it to see the CSS classes and the HTML structure that Struts uses to render the message. You can also look in the template files.
By default, struts displays each action message as follows:
<ul> <li><span class="actionMessage">${message}</span></li> </ul>
Each message will have <li><span class="actionMessage">${message}</span></li>
.
You can create CSS for actionMessage or change the template file for rendering as you want.
Template files for them are located in:
/template/simple/actionerror.ftl /template/simple/actionmessage.ftl
Field error can also be useful for you:
/template/simple/fielderror.ftl
Note: if you use the xhtml theme, these files can be located in this folder in the template
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