No, you’d need a filter or action hook to put code on functions.php. We’ve no way of knowing if that’s possible or not with your theme, hence why you were referred to the theme’s support channel. They’d have a better idea than anyone.
If your theme has a category.php template, you could try placing the offered code in a promising looking location. If not, you need to identify the actual template used and use conditional code to only output the description if the queried object is a category term.