You can use layout: decorator and layout: fragment so I understand you want the contents of the screens to be changed but without changing what's around the menus as examples.
In your "Main.html" screen you should put a layout: fragment as in the example:
<div layout:fragment="conteudo">
<p>Conteúdo principal</p>
</div>
In this place will be injected the content that you put in the other pages. In the other pages you will use a decorator, in this case I called "Main" and implement the "Contents" of the layout: fragment, example:
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:th="http://www.thymeleaf.org"
xmlns:layout="http://www.ultraq.net.nz/thymeleaf/layout"
layout:decorator="Principal">
<head>
</head>
<div layout:fragment="conteudo">
Pagina 1
</div>
</html>
You can look at the complete documentation at link