Comment (by Roland Haas):

I added CSS code like this to /usr/lib/mailman/templates/site/en/article.html which may help with newly archived postings. Old postings require either regenerating all cached HTML files using bin/arch --wipe or editing the files html in /var/lib//mailman/archives/public/users/ which might be saver in that it guarantees that the message numbers will not change (https://mail.python.org/pipermail/mailman-users/2016-November/081557.html).

--
Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/723/archived-emails-to-mailing-lists-are