My intention is to add a vertical bar to IDLE to indicate preferred line length at column 80.
I have tried to find a configuration option for the Text tkinter widget that would allow this but have found nothing.
I was hoping it would be a simple configuration option so I could just add a another item the text_options dictionary within EditorWindow.py found within Python\Lib\idlelib.
I am not sure how styles/themes work but do they have the capability to change the background colour of only 1 column in a Text widget?
Outline of possible solution:

Create a 1-pixel wide Frame, with a contrasting background color, as a child of the Text.
Use .place() to position the Frame at an appropriate horizontal coordinate.

Possible issues:

I don't see any easy way to get the coordinate for a particular column.  Text.bbox("1.80") looks promising, but doesn't work unless there actually are 80 characters in the line already.  You may have to insert a dummy 80-character line at first, call update_idletasks to get the Text to calculate positions, call bbox to get the coordinates, then delete the dummy text.  Repeat whenever the display font is changed.
The line would necessarily appear on top of any text or selection region, which isn't quite the visual appearance I'd expect for a feature like this.
Adding a vertical bar or other marker to tkinter Text widgets at a particular column