There are many reasons why someone could want to change the default font of their computer’s operating system. A popular request for this feature is for the inclusion of the OpenDyslexic font (found here) for better accessibility for students.
In the most recent update to Mac OS Seirra, users are unfortunately NO LONGER able to change their system’s font. Previously this used to be a function available (through detailed administrative modification) but now it is systematically barred off via Apple's coding. Changing the main font of the computer is a complex process that would break the OS if it were attempted. While we regularly encourage explorative optimization of your computer: this is one we are unable to support.