app/views/layouts/email.html.erb
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="viewport" content="width=device-width"/>
<%= stylesheet_link_tag "email" %>
</head>
<body>
<div class="container" style="margin: 20px 40px">
<%= yield %>
</div>
</body>
</html>