It's in mail options in settings. This article may be of help
although it does not have any keyboard commands. If you still have
trouble, I would recommend contacting the Microsoft accessibility
help. I remember that it was also difficult for me to locate.
Here is the link to the article
https://support.lesley.edu/support/solutions/articles/4000048040-how-to-change-your-office-365-email-to-from-the-light-version
Lastly, the number to Microsoft accessibility is (800-936-5900).