How do I change fonts and add colors in Contacts?
The Contacts app uses the system font. To change font or font size in Contacts.app, you must change it for your whole system. There is a way to change the system font from San Francisco font to Lucida Grande font which may be more appealing to you. There is a small app that lets you pick between San Francisco and Lucida Grande fonts for your system. This web page provides a link to the application or you can go straight to it here.