Thanks @mbc66 - took a lot of working out but I’ve got there.
These are the steps I took:
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Download the first item (‘AxQuery and Javascript 1.03.rp’) from the top post on this thread
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Open ‘Examples’ and copy the Javascript engine section to the footer of my prototype (so it appears everywhere, rather than copying to every page)
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Remove the Javascript box and edit the style box, so I end up with this in the footer:

Finally, in the footer on page load I hide the Javascript Engine:

Very hacky but this works for me as a way to insert custom CSS locally - in this case, for font smoothing.
Thanks for your help.