Is there a way to change to a particular font style using a single function key? For instance, I would want to be able to change any font to "Helvetica Bold 16" every time I press F5 if rich text is selected, regardless of application.
You should try and follow the instructions in this answer, How to create Automator Service to change font of selected text?, I checked it and it works. Follow the assign a shortcut to a script link, https://superuser.com/questions/153890/assign-a-shortcut-to-running-a-script-in-os-x/264943#264943, then change Menlo to Helvetica Bold and 18 to 16 in the script provided there. Hope this helped. You can comment here if you run into issues and maybe I can help.