Hi J Hills,
Thanks for contributing to our forum. It is not our intent to make it so certain templates cannot be customized, but I was able to reproduce the behavior you described with respect to customizing the “Retro Winter” test and have passed the details of the issue over to our technical team to make them aware of the issue.
We updated the editor that is used on the template customization page within the last year and it appears this template must contain some code that is being processed differently by this new editor.
I am not sure what may be necessary to resolve this particular customization issue, but we will look into the possibility of addressing it with a future site update and I will make you aware of any new information which becomes available on the topic through this thread.
We apologize for any inconveniences this may be causing. If you need any further assistance with your account, please feel free to contact our Customer Support team by mousing over the “Help” tab and selecting the “File Support Case” option.
-Mike