Whereas to me, 'not shaded' is easily the worst.  Users can customize the sidebar colors, but since the default is the same at the text, I would not want to deliver IDLE with a scheme that I think is awful.

On Windows, I agree that the light theme border is too heavy.  The dark  theme light border is barely noticeable.  On mac, its the opposite: light theme ok, dark theme white border is too prominent.  I would be willing to look at a change.