In live code demos, the visual weight of the line numbers is heavier than the code next to it.  Instead, is should be light gray, just dark enough to easily locate a line of interest, and light enough to tune out while reading the line and surrounding code.   The current weight impairs readability.

The line number sidebar colors are already user configurable.  We expected preferences for this to be as diverse as for anything else.  I changed them on my custom dark theme to dark blue on light blue, but I don't expect anyone else to like that.

Notepad++ black on white uses a slightly lighter gray on a very light gray (RBG about 233 versus 255 for white).  I think a background different from the text is a good idea.  In retrospect,  I might have suggested a default something more like N++,  but I am not currently willing to change the existing default.

I agree that the line numbers should be less prominent.

Terry, why not change the the default? ISTM we should strive to have good default colors, they are what most users use and see.

I also think we should improve the dark theme, but that's a separate issue.

When the question of changing default-x.def has come up before (and there are things I would like to change) the answer has generally been that we should not, a) because it would be too disruptive, including affecting exiting customizations, and b) because users already have the option of customizing.  Most of the behaviors we change are not customizable, so we have to change the current behavior to give most people a better behavior, even if some people (including Raymond) object.

However, I will reconsider here since a) Tal, who I believe choose the current default, not only agrees with changing it by also the direction, b) line-numbers are only a year old, and, importantly, c) themes are saved as a whole, rather than as individual pairs, so that existing customizations will not be affected.  Even so, I expect that there will be people using IDLE Classic that prefer the current default.

If we are to change, we need specific foreground and background RGB numbers to replace the Current ones.  Raymond, Tal, and Cheryl if you want, please play with the line number (and future Shell sidebar) colors and propose something specific for general use.  (Raymond, I can imagine that you might prefer for projection with a particular system something different than you think is best for default screen use.)  I intend to post to idledev and maybe elsewhere to get other opinions (including for IDLE Dark).  Since I will not use whatever is chosen, my main concern is that we not choose something so light as to be unreadable by people like me with less than 'perfect' vision.

Related issue: the IDLE New theme was added years ago as a copy of IDLE Classic.  I presume the intent was to provide an alternative (light) default, but it was never modified, making it currently useless.  We should either delete it, while recognizing the name, or actually change it.  I have some ideas for the latter, but I presume Raymond intends this issue to be about changing IDLE Classic.