Open IDLE by running from console or importing idlelib.idle in interactive Python window.  In the IDLE Preferences dialog, create a new custom theme or key set.  [Apply] or close with [OK].  Reopen, if necessary, select the new custom set as current, select Delete, and Apply or close.  IDLE will delete the set and then try to read it.  A warning message will appear in the console/interpreter for each item in the set that is had to replace with the default value.

IDLE should replace the deleted set with one of the builtins before reading, or even better, allow and require deletion of something that is not current.