Here we say, if the {{content}} placeholder contains "font-family:'Lucida Sans Unicode','Lucida Grande','Tahoma',Verdana,sans-serif;", then go ahead with these replacements, otherwise, just display the default content. Because the above font stack is used for the paragraph elements of comments, this will apply to only those elements.

content_font_replace_zd

{% if content contains "font-family:'Lucida Sans Unicode','Lucida Grande','Tahoma',Verdana,sans-serif;" %}
{{content | replace: "font-family:'Lucida Sans Unicode','Lucida Grande','Tahoma',Verdana,sans-serif;", "font-family: Georgia, Times, "Times New Roman", serif;" | replace: "font-size:14px;", "font-size:12px;" | replace: "color:#2b2e2f;", "color:#6b6d6e;"}}
{% else %}
{{content}}
{% endif %}