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As a follow-on to #24820, when a particular theme is selected in the configuration files, but that theme is not available, IDLE will print out a bunch of warning messages on console. That can occur for example when using a newer built-in theme in an older version before that theme was added, or if the user highlight config file was modified or removed.
IDLE should be smart enough to recognize that and switch to a default theme, without dumping out console error messages. May alert user to that case with an appropriate error message, e.g. theme xyz not found, using default.
The current workaround in #24820 (when a newer theme is chosen in a new version, explain it may not work in older version but explain how it could be made to) is unfortunately necessary unless a more clever solution for this backwards compatibility issue can be found.
Would apply to keysets, etc. in the same was as themes.