I just tested saving the global RSVP Pro params and a specific template and a style tag such as
<style type="text/css">.custom-style {color:red;}</style>
is not stripped out.
Perhaps it is your WYSIWYG editor that is doing this???
In JCE there is a setting to allow inline style elements in the editor profile settings
re {EMAIL} if you edit the file administrator/components/com_rsvppro/libraries/inviteehelper.php and insert this code
if (isset($currentinvitee->email_address) && $currentinvitee->email_address != "")
{
$message = str_replace("{EMAIL}", $currentinvitee->email_address, $message);
}
else if (isset($currentinvitee->user_id))
{
$iuser = JEVHelper::getUser($currentinvitee->user_id);
$message = str_replace("{EMAIL}", $iuser->email, $message);
}
after
$message = str_replace("{USERNAME}", isset($currentinvitee->username) ? $currentinvitee->username : $name, $message);
at line c. 1231 it should replace {EMAIL} in your invitation messages.