There is a way to create theme within Mautic … sort of … still require to use a desktop code editor
Download my blank MJML theme over at Blank MJML Theme and create an email from it then save it.
After saving it, open the code box and copy all the MJML code from it and paste it in your favorite desktop editor. I use Visual Studio Code.
Create a theme file from it and do a search / replace to edit all the image path if you are going to include images in the theme folder. You would also need config.json file and thumbnail in the theme folder before zipping it up.
Or you can do what @mzagmajster mentioned and create an email in Mautic from my blank MJML theme then save it. When you need to use it again, clone it and make whatever change in the cloned version and save it.