Hopefully this will help, but I think I understand what you’re doing. I’m not exactly sure on the details of how to accomplish it, but I can give you a general outline.
When your field is loaded or changes the script is run.
The script will look at the field and get the font that needs to be include then creates a new <style> element in the head of your document with the link to the correct CSS file.
You may also want to delete any previous <style> elements that were created when the user switches fonts.
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