How to change font for the CodeMirror editor

Just to follow-up on the accepted answer, the version of CodeMirror I'm using (5.55.0 at the time of posting) requires a wildcard:

.CodeMirror * {
/*          ^
*/
  font-family: Arial, monospace;
  font-size: 16px;
}

Try setting the CSS on:

.CodeMirror {
font-family: Arial, monospace;
font-size: 16px;
}

This selects the element that contains all the formatted code.