The exception message says that your problem is a bad byte in one of the config files.  The most likely culprit is the one you edited, which I presume is ~/.idlerc/config-highlight.cfg.  "Invalid start byte' suggests that it tried to decode as utf-8, but you used a non-ascii char and saved with some other encoding.  I suggest you stick with ascii only for theme names.  The set of theme item names should match the all-ascii set used in idlelib/config-highlight.def.

For future reference: a crash is a segfault or Mac equivalent, without a python traceback.  Uploaded text files should be plain ascii or utf-8 text, uncompressed and not wrapped.  "I have a problem.  Help me" requests should normally go to python-list or other forums.  The tracker is for bug reports and enhancement requests.

In this case, I decided to make this a bug and enhancement issue and changed the title accordingly.  The bug is that IDLE stopped instead of continuing without the user configuration, the same as it would if there were no file.  I propose to catch the exception and replace the traceback with the file name and error (the enhancement).  At least for user config files, IDLE should then continue (the bugfix).  The revised message will be something like the following.

"Unable to read .../.idlerc/config-highlight.cfg.
UnicodeDecodeError: ...
IDLE will continue without this user config file."