Hi,
Exactly the same way you do in regular html - use <style> tags.
MailBuilder builder = new MailBuilder();
builder.From.Add(new MailBox("alice@mail.com", "Alice"));
builder.To.Add(new MailBox("bob@mail.com", "Bob"));
builder.Subject = "This is HTML test";
builder.Html = @"
<html>
<head>
<style>
body {background-color:lightgray}
h1 {color:blue}
p {color:green}
</style>
</head>
<body>
<h1>This is a heading</h1>
<p>This is a paragraph. <img src = 'cid:image1' /> </p>
</body>
</html>
";
// Read attachment from disk...and add it to Visuals collection
MimeData image = builder.AddVisual(@"c:\image.jpg");
image.ContentId = "image1";
IMail email = builder.Create();
using (Smtp smtp = new Smtp())
{
smtp.Connect("server.example.com"); // or ConnectSSL for SSL
smtp.UseBestLogin("user", "password");
smtp.SendMessage(email);
smtp.Close();
}