I see the 3.4.4 is not an immediate concern, so that's good.

FYI, I get the (multiple) error messages on console consistently on Mac, but it probably depends how it was launched. Agree the code for future versions should do a better job to detect the theme isn't present and switch back to a default theme rather than just puking out error messages as it tries to get each element of the non-existent theme. :-)

I still think the approach I took may be viable. It doesn't require changes to old versions, and relies on the fact that a user theme with the same name as a default theme is effectively hidden by the default theme. 

If a 'new' theme is selected, it's always written out to config-highlight, so it will be present for older versions (not errors and the internal default colors). Given that, I need help understanding what the 'name' vs. 'name2' etc. solution gives us?

(Not being able to write .idlerc wouldn't be an issue, because then you wouldn't be able to write out the change to say that is the theme to use anyway).

The only drawback I see is that a user running a previous version can customize IDLE Dark and those changes would not appear when using a newer versions of IDLE that had IDLE Dark as a default.

This would negatively affect only those users who change to IDLE Dark, actually modify the theme, and switch back and forth between versions of IDLE. The consequences of this occurring are only that the changes don't show up. In other words, I feel this affects a very tiny number of people, in a relatively minor way.

The existing warning on choosing themes will be encountered by anyone who might switch themes in the new version, and particularly for the non-hardcore, would be at the least surprising if not confusing or alarming. I think it's safe to assume that far more people try existing themes vs. create custom themes and use multiple versions of IDLE.

Given the very minor consequences and how few people it would affect, vs. how many people would encounter that warning and the impression it would leave, this seems a case of I'd rather be unobtrusive and 98+% correct then obtrusive and 100% correct. 

If it were my product, there's no question I'd drop the warning under those circumstances, and if it were a choice between including the warning or not including a new theme, I'd personally choose to not include the new theme. 

Anyway, it seems we have time to think more about this...