But there is something _I_ can do now. :-)

I put a pdb.set_trace in that code you quoted, and went exploring. And here is what I found.

It calls
    currentOption = idleConf.CurrentKeys()
That one calls
    return self.current_colors_and_keys('Keys')
and _that one_ calls
    default = self.GetOption('main', 'Theme', 'default',
                                      ^^^^^ note this
                                 type='bool', default=True)
and later it uses defaut as default for both theme and keys.

I did have custom keys, but not a custom highlight theme. Sure enough, as soon as I made a trivial copy of IDLE classic, things worked perfectly. :-)

So now I don't have a problem anymore. But still I think it is a bug that should be fixed. Not everyone who hates Alt+p also hates the orange words on white background. :-D