How to get rid of new default Version 11 and 12 Input/Output fonts -> and keep the traditional Courier font

No need to modify a stylesheet, it is sufficient to evaluate

SetOptions[$FrontEnd, StyleHints -> {"CodeFont" -> "Courier"}]

or equivalently

CurrentValue[$FrontEnd, {StyleHints, "CodeFont"}] = "Courier";

for permanently setting the default font to "Courier New" system-wide in Mathematica 11. Note that the undocumented StyleHints option is new in version 11, it isn't recognized in the previous versions.

Following the lead from J.M., and then some comments therein from User18 and QuantumDot, the following is working:

In the common StyleSheet, under Notebook options, I added the option:

StyleHints->{"CodeFont" -> "Courier"}

So, for example, the options now read:

Cell[StyleData["Notebook"],
 Saveable->False,
 ShowAutoSpellCheck->False,
 StyleHints->{"CodeFont" -> "Courier"},
StyleMenuListing->None]

This works! Many thanks.