Pretty sure that’s in app/views/user_notifications/digest.html.erb. Certainly the digests I get from padpors.com are filled with things like (excerpt still in quoted-printable mail format):
<table dir=3D"rtl" style=3D"width:100%;background:#f3f3f3;padding:0;borde=
r-spacing:0;font-family:Helvetica,Arial,sans-serif;font-size:14px;font-we=
ight:200;line-height:1.3;vertical-align:top;">
Which is remarkably similar, except for the dir="rtl", to the lines in digest.html.erb. Fix the font-family usage in your copy of the file, like you did for the RTL fix.