I created a firefox theme to use for myself. It works when I use it temporary on site “about.debugging”.
However, I could not manage to create an installable file to use it permanently. I zipped the manifest.json file as described here
I also tried what is described here for “sideloading”. The attempt to sign the “addon” resulted in following error:
Sorry, we couldn’t load your WebExtension.
Fehler: Validation was unable to complete successfully due to an unexpected error.
Has anybody an idea?