Hello Pablo,
I really recommend to use HTML in the email, Then use the WYSIWYG Editor online (e.g. http://htmleditor.in/index.html ) to get the HTML Code with the CSS Embebbed. Use the $$variable$$ to add variables to your html (Don't forget to add the .html file in the WEB-INF WebRatio Explorer Project Folder) the Assing the HTML to the Email Unit, and when you start mapping the Parameter you will find the $variables$$..
Good Luck!