Hello @PaulP14 ,
I can add your account to the list of customers interested in having a border system added to the 3ge emails. At this time, if you're wanting borders, you'll want to check out some of the pre-built templates that are found on the new template selection screen. Their colors can be controlled by using the default coloring options found in the Design tab of the editor.
I can, your name has been added to this feature request. Thank you for your feedback.