Maybe the easiest way is to add some custom css
i {font-style:italic; }
If needed, you could add the ‘important’:
i {font-style:italic !important; }
If your theme supports ‘extra/custom css’, put that in there. Or, create a child theme and put it in that child theme’s style.css .
Not recommended to put it in the theme’s style.css file, as a theme update would overwrite your change.