Hi,
There is no supported way of adding fonts to the web client. So far, is either:
1. Compose mail on a external document software like libreoffice, copy/paste to body e-mail.
2. Manually add the fonts to the web application and packaging it again.
Does anybody knows where to find a guide on how to perform "2. Manually add the fonts to the web app...".
Also, there is this bug #62035 ( https://bugzilla.zimbra.com/show_bug.cgi?id=62035 ) filled for a functionallity enhancement. If you are interested in this functionality, it would be a good idea to leave your vote for it and CC.
Thanks,
Seb