FengOffice 1.7.5
File:
application/controllers/MailController.class.php
Change Line 298:
$css = "font-family:Arial,Verdana,sans-serif;font-size:18px;color:#222;";
FengOffice 2.0.x
File:
plugins/mail/application/controllers/MailController.class.php
Change Line 334:
$css = "font-family:Arial,Verdana,sans-serif;font-size:18px;color:#222