Please note unlike web browsers, email clients like Outlook only supports a subset of Styles etc.

For more information see article Word 2007 HTML and CSS Rendering Capabilities in Outlook 2007 (Part 1 of 2) | Microsoft Learn.

One example of where you might want to add custom styling, is to an embedded Sysero Report inside the Email Template e.g.

1. In th email template go to Tools > Source Code.
2. At the top add <style></style> tags.
3. Inside the style tags you can add things like .gvTable td {border:1px black dotted !important}, to say add a border around the results grid.

One example of not being supported is td:nth-child(n), where you can select a column.