Message 269692 - Python tracker

Seems to be working correctly now, so I will review the code in detail.  I will look into extracting a common get_theme_keys function and editing the long docstring currently attached to CurrentTheme.  I want to look at paired functions in configdialog, such as def VarChanged_builtinTheme/Keys to see if there are any unnecessary differences.

The hardest thing is adding tests.  Correct answers depend on the configuration vales, both default and user overrides.  See #24737 for the latter.