Before looking at this, I ran the leak tests and found a worse problem in the test suite.  I opened #27044 and fixed the problem.

I did not get the 'theme changed' message (msg220243) when running the leak test in the console, but did when running it either the file or test_file as __main__.  In the former case, the process is started by test.regrtest, which ends up calling unittest.  This is how buildbots run.  In the latter case, the process is started by the idlelib file, which calls unittest.