Hello, @oscardc,
In order to solve your issue please watch and follow the steps shown in the video: https://www.youtube.com/watch?v=PcMBPYAMHnw
Further, you can check this article: https://wpfullcare.com/wordpress-theme-editor-missing-how-to-fix-it/
Hope this will help you in solving your issue.
Regards 8Themes Team.