There are two ways to do this:
I took the text position "Caught from some unhappy master..." as an example.
1. Change it in the original html file. When you look in the original html, you will find:
<pre xml:space="preserve">
Caught from some unhappy master whom unmerciful Disaster
Followed fast and followed faster till his songs one burden bore—
Till the dirges of his Hope that melancholy burden bore
Of "never—never more!"
</pre>
Where the definition for <pre...> is on top of the file in the style section:
pre { font-style: italic; font-size: 90%; margin-left: 10%;}
You can change font-style: italic to what ever you want within the html file.
Or
2. You open your converted eBook (epub or azw3) with calibre editor, search for the text and look with the live css panel what apply to the tag and change the this rule(
http://manual.calibre-ebook.com/edit.html#id31).