I found this way not sure it works for all the CSS primitives
By setting the header property "Content-Type" to "text/html"
mimeMessage.setHeader("Content-Type", "text/html");
now I can do stuff like
mimeMessage.setHeader("Content-Type", "text/html");
mimeMessage.setText ("`<html><body><h1 style =\"color:blue;\">My first Header<h1></body></html>`")
Regards