Besides this, what style do you want to use? Is there a journal or publisher you're preparing the manuscript for?

If this is a general requirement for Emerald Harvard, we can fix it for you. Where does it say to use et al in italics?

Otherwise, find
<macro name="author-short">
<names variable="author">
<name form="short" and="text" delimiter=", " initialize-with="." delimiter-precedes-last="never"/>
<substitute>

Change to
<macro name="author-short">
<names variable="author">
<name form="short" and="text" delimiter=", " initialize-with="." delimiter-precedes-last="never"/>
<et-al font-style="italic"/>
<substitute>

General instructions here: https://www.zotero.org/support/dev/citation_styles/style_editing_step-by-step

It looks like this is a general requirement for Emerald Harvard:
http://www.emeraldgrouppublishing.com/authors/guides/write/harvard.htm

Italics also used in a handful of articles in different journals I checked.