It looks like the solution I wanted was already incorporated.
Ran "make html-nn" and the file /stylesheets/nn/gimp-help-custom.css was
copied to the html folder. Thereafter ran "make html-en"
and the file stylesheets/nn/gimp-help-custom.css was not copied.
I have not tested it on other languages yet.
Sorry for being so incompetent. (But it led to an interesting discussion).
(My internet connection was down today (again), therefore a bit late).
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