You can add a CSS to every email / for an individual user email account / email template / or specific email.
1. System wide email CSS - Once you add a system wide email CSS all new emails will use this style.
Go to : Admin > Setting > select tab named 'Email' and set your CSS
With this example the email editor for all the users would look like the below.
2. User account email CSS - Your
Support for XPath as a standard field type has been added. When a field is marked as XPath then the XPath syntax highlighter will be used by default and the field value will be validated to be a valid XPath expression.
XPath syntax editor
HTML Editor
JavaScript Editor
XML Editor
CSS Editor
Style sheets can be specified at the site or page level and based on the browser filter.