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
Each CMS site could have one default article template, you can set it in the Admin tab of the site data entry screen.
Select Handlebars as the default article parser, then enter the handlebar template
When you insert an article compoent in a page, it will look like:
Then you can add your style sheet to change the looking of the article