Check out example codes for "css system default font". It will help you in understanding the concepts better.

Code Example 1

/* System Fonts as used by Medium and WordPress */
body {
  font-family: -apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Oxygen-Sans,Ubuntu,Cantarell,"Helvetica Neue",sans-serif;
}

Learn ReactJs, React Native from akashmittal.com