Use Case:

Rather than customize CSS for each email template, the user would like to define a custom_value to override CSS within an email template.

This can be done using {{custom_values}} merge fields.

However, that merge field is not rendered when building a new template, and no CSS customizations are applied to the email template.

Please update the Email Template to process any merge fields within the Custom CSS field so users are able to see the applied CSS.

This should also render / process the custom_values when using the Email Preview feature.