[RbnJrg]

I think I found the final solution with the following css code:

h2 {
   height: 2em;
   border-bottom: 3px solid;
   margin-top: -1em;
   margin-bottom: 1em;
   font-size: 1.4em;
   text-align: center;
}

.text {
   float: left;
   width: 20%;
   line-height: 2.8em;
   font-size: 1.4em;
   margin-left: 40%;
   margin-right: 40%; 
   background: white !important;
}

p {
   text-align: justify;
   text-indent: 0em;
}

This is the look in Calibre Viewer:



And this is the look in Kindle Previewer (Kindle Fire - Night Mode):



Below I attach the respective epub.

Regards
Rubén