Ok, so here are the results. As a noob, I thought callout box were standards. It appears that it is not. I checked in the CSS of the Simple theme V2.2.0.0, I did not find something that refers to callout box or alert. So if anyone wants to achieve callout box, here is a solution :
In Publii>Tools>Custom CSS, you add the following code, courtesy of Bootstrap :
.alert {
position: relative;
padding: 0.75rem 1.25rem;
margin-bottom: 1rem;
border: 1px solid transparent;
border-radius: 0.25rem;
}
.alert-heading {
color: inherit;
}
.alert-link {
font-weight: 700;
}
.alert-dismissible {
padding-right: 4rem;
}
.alert-dismissible .close {
position: absolute;
top: 0;
right: 0;
padding: 0.75rem 1.25rem;
color: inherit;
}
.alert-primary {
color: #004085;
background-color: #cce5ff;
border-color: #b8daff;
}
.alert-primary hr {
border-top-color: #9fcdff;
}
.alert-primary .alert-link {
color: #002752;
}
.alert-secondary {
color: #383d41;
background-color: #e2e3e5;
border-color: #d6d8db;
}
.alert-secondary hr {
border-top-color: #c8cbcf;
}
.alert-secondary .alert-link {
color: #202326;
}
.alert-success {
color: #155724;
background-color: #d4edda;
border-color: #c3e6cb;
}
.alert-success hr {
border-top-color: #b1dfbb;
}
.alert-success .alert-link {
color: #0b2e13;
}
.alert-info {
color: #0c5460;
background-color: #d1ecf1;
border-color: #bee5eb;
}
.alert-info hr {
border-top-color: #abdde5;
}
.alert-info .alert-link {
color: #062c33;
}
.alert-warning {
color: #856404;
background-color: #fff3cd;
border-color: #ffeeba;
}
.alert-warning hr {
border-top-color: #ffe8a1;
}
.alert-warning .alert-link {
color: #533f03;
}
.alert-danger {
color: #721c24;
background-color: #f8d7da;
border-color: #f5c6cb;
}
.alert-danger hr {
border-top-color: #f1b0b7;
}
.alert-danger .alert-link {
color: #491217;
}
.alert-light {
color: #818182;
background-color: #fefefe;
border-color: #fdfdfe;
}
.alert-light hr {
border-top-color: #ececf6;
}
.alert-light .alert-link {
color: #686868;
}
.alert-dark {
color: #1b1e21;
background-color: #d6d8d9;
border-color: #c6c8ca;
}
.alert-dark hr {
border-top-color: #b9bbbe;
}
.alert-dark .alert-link {
color: #040505;
}
Now, if you want to insert a callout box, from the editor, you wrap your paragraph in a html tag, for example <p></p> with the desired callout box as a class. For example, if I want a blue-ish callout box arround my paragraph, I add in the editor :
<p class="alert alert-primary">
My paragraph
</p>
and it works \o/.
If you want some other kind of callout box, here is the bootstrap callout box related documentation :
https://getbootstrap.com/docs/4.4/components/alerts/
Enjoy !
