Tokens that are rendered through the theme layer, use the wrong theme in test mails

Problem/Motivation

When sending the test emails, certain token replacements are rendered through the active (backend) theme instead of the default theme (frontend).
This can cause different HTML between the test mail and the actual live mail.

Steps to reproduce

Create an email with the token [commerce_order:order_items]. (probably other tokens have the same issue, but this is the one that tripped us up)
Have a backend theme setup (for example: seven)
Have a frontend theme (for example my_custom_theme)
Place an actual order and observe that the token gets replaced with the content from twig files in "my_custom_theme".
Now send a test mail for the same order and observe that the token gets replaced with the content from twig files in "seven".

Proposed resolution

Remaining tasks

User interface changes

Forced rendering/sending test mails through the default (frontend) theme.
OR
Have an option to select a theme when sending test mails.

For the actual sending of mails: this can/should be handled by the implemented mail solution. We are using symfony_mailer for this and it works fine for the mail itself. The problem is with the generated content from the test mail, that contains rendered content from the admin theme...