Schedule update: 3.4.4 will be delayed untill after 3.5.1, which will be much sooner than the original minimum of 4 months after 3.5.0.  Larry promises at least two weeks notice before 3.5.1rc1 but will give no hint of when notice might be given.  So we have at least two week to do something correct.

I am against duplicating names because it is wrong in principle.  On the other hand, I understand the objection to a popup always and forever.  It is past time anyway to enable a working [Help] button, #22726.  The current popup help text should go there as tab-specific text.  The only automatic notice on selecting a new builtin theme would be 'New theme, see Help', maybe in orange (warning color, but not emergency red ;-) added in the empty space below.

I believe one new 'name2 = new_theme' addition to config-main would be enough for future additions.  We add to configHandler 'new_themes, a set, currently {'IDLE Dark'}.  If a selected builtin is in the set, the message is displayed and name2 is written.  On reading, if name2 is present but not recognized, revert to IDLE Classic, which is present on all systems now.

Intended result: all IDLEs keep working, new releases get a minimally intrusive notice pointing to a full explanation.