Hi,
Could you please specify how do you define the newsletter's html code?
I don't know if that's the case here, but please verify whether you have included <head> section in your newsletter code.
This section is important for our controls and scripts that later insert the stylesheet definitions into this <head> section.
We've come across a similar issue where this was the cause.
To solve it, we put the default header text from our sample newsletter:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Newsletter</title>
<meta http-equiv="content-type" content="text/html; charset=UTF-8" />
</head>
<body>
and it helped. So you may try it as well.
If this wasn't the cause in your newsletter, please let us know.
Regards,
Zdenek