Th:replace doesn't accept the path if I generate it but works if it is hardcoded

[Mike Grabl-j]

<p th:text="'themes/'+${theme}+'/general/footer::footer(theme=' + ${theme} +')'">test</p>
<footer th:replace="themes/holmes/general/footer::footer(theme=holmes)">Footer</footer>
<!--<footer th:replace="'themes/' + ${theme} + '/general/header::header(theme=' + ${theme} + ')'">Footer</footer>-->

This code works. That text in < p > is printed here for debug purpose. The variable theme contains 'holmes'. The code produces this HTML fragment:

<p>themes/holmes/general/footer::footer(theme=holmes)</p>

Well, I hoped the commented code would work. But no, it doesn't.

This exception occurs:

org.thymeleaf.exceptions.TemplateInputException: Error resolving template ['themes/' + ${theme} + '/general/header], template might not exist or might not be accessible by any of the configured Template Resolvers (template: "themes/holmes/posts/postList" - line 14, col 9)

Could you help me understand why? And how to fix?