Hi, when you upload, you need to have those files in the zip you are uploading.
Upload it by using the Settings - Themes - New option in Mautic.
Just download any theme and see how yours is different.
More info:
I see the exact same issue in version 4.3.1. When I download an existing theme by the Mautic team, change one minor thing in the email template, then zip in the exact same structure and try to upload and install, I still get this error:
The theme you tried to install is missing the following required files and thus could not be installed: config.json, html/message.html.twig
Can you confirm that the file extenstions are properly saved, your editing software didn’t change it?
If it’s all okay, plz send me in PM, I’ll take a look
@joeyk, apologies for my slow follow-up. Windows adds a level to the zip folder when you rezip; blank/blank/etc. If you watch out for that the errors disappear.