You can probably achieve this in a plugin by doing something along the lines of:
- with the plugin API add a
api.addSaveableUserOptionField("composer-custom-font")
- add the field to the
UserOption
table in the db and the serializer so it can be set - add a connector to the user’s interface page so the setting can be set
- you can make the setting a dropdown, for example, so if the user picks a certain font/font style it adds their preference in the db
- you can extend the composer code to take into account their selected preference, and either change the styles directly with JS or make it add a CSS class based on the preference chosen, which you can apply styles via CSS for that specific class