How do I change the font in an email sent from the contact panel in Zoho Books?

How do I change the font in an email sent from the contact panel in Zoho Books?

I send clients email notes and like the ability to send them right from the contact entry. 

Unfortunately, I can't find a way to change the default font from Times to Arial. I would like all my emails to match, and have changed the templates for invoices, estimates, etc., I just can't figure out how to change this type of email. 

Suggestions?  Thanks.