[PeterT]

While yes, you can code

<p style="text-align:center">* * *</p>

it's being suggested that using a stylesheet and named styles would be more beneficial.

You would want to make a new stylesheet (say my.css) and it would contain

.center {
    text-align: center;
}

Then you would link my.css to the files within the eBook within Sigil, and in the actual files just code

<p class="center">* * *</p>
<h2 class="center">Préambule</h2>

Or if you wanted ALL <h2>'s to be centered you could place in my.css

h2 {
    text-align: center;
}

and then just code

<h2>Préambule</h2>