th:text works with an expression, but th:replace doesn't - thymeleaf

Question

Variable
From the controller I sent:
model.addAttribute("theme", "holmes");
html
<!--<p th:text="${'themes/' + theme + '/general/footer::footer(theme=' + theme +')'}">test</p>-->
<footer th:replace="${'themes/' + theme + '/general/footer::footer(theme=' + theme +')'}">Footer</footer>
Exception
org.thymeleaf.exceptions.TemplateInputException: Error resolving template [${'themes/' + theme + '/general/footer], template might not exist or might not be accessible by any of the configured Template Resolvers (template: "themes/holmes/posts/postList" - line 25, col 9)
Proof that the commented expression works
Problem
The commented out expression works. But the uncommented one blows up.
To the best of my ability I understand that the compiler sees a string rather than an expression here.
I am keen to understand:
What the difference is between the commented and uncommented lines.
How to write the replacement expression correctly.

Answer 1

I think you are very close. This works for me:
<footer th:replace="'themes/' + ${theme} + '/general/footer' :: footer(theme=${theme})">Footer</footer>
The reason this works is (probably) because of the way the fragment specification syntax is evaluated:
templatename::selector
Each part on either side of the :: needs to be handled as a separate component. Putting the :: in the string (as in your example) misleads the Thymeleaf renderer. I asssume it tries to find a template whose name is the entire string (including the ::) and the selector part is lost.