[GeoffR]

There is actually no simple way to change scene breaks that is guaranteed to work for all books, because some publishers who use a space to mark scene breaks might also use the exact same style for spaces that are not scene breaks, and there is no certain way to know which are supposed to be scene breaks and which are suppposed to be something else without reading the book.

But assuming the publisher has used a unique style for scene breaks, say something like:

<p>... last line of previous scene.</p>
<p class="newscene">First line of next scene ...</p>

where p.newscene is something like:

p.newscene {
  margin-top: 1.5em;
  text-indent: 0;
}

Then you can search the html for:

</p>
<p class="newscene">

and replace it with something like:

</p>
<h6 class="myscenebreak">* * *</h6>
<p class="mynewscene">

and add h6.myscenebreak and p.mynewscene to the stylesheet:

h6.myscenebreak {
  font-size: 1em;
  font-weight: normal;
  margin-top: 1em;
  margin-bottom: 1em;
  text-align: center;
}
p.mynewscene {
  text-indent: 0;
}

However there are a million different ways the publisher can do the scenebreaks, the above is just one example, so you need to treat every book as a special case.