Shortcut to Change to a Specific Font
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.