In D8 how could I add a body class to a node create or edit page in a custom theme?

Am thinking there should be a way of doing this via the preprocess_html function in my theme file.

Please note that I'm trying to add a class to the body of a node create or edit page, not a normal node page. This is a very specific thing I'm trying to do which is not met by any previous answers.

I'm hoping that there is some variable within the preprocess_html function that can target these pages.


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