An error occurred while saving the commentErik commented
For those of you who wish to create a custom stationery based on html, I found out that it is as simple as this:
Open a new message. Click on HTML to the right in the format tab, and you will see the current code in a new window. Simply replace the code with your own.
I don't know if that was the issue for this thread, but at least it solved my problem. In Apple Mail it is a lot trickier.