I re-read #7949 and read the problem as mixing a hard-coded (by tk or Idle is not clear) background (white) with a system-default foreground, which is also white when the system background is black.  Hence the screenshot in duplicate #16984.  In any case, that issue is complementary to this one, about user-configured tagged text colors.

Parsing an existing format is an interesting idea.  Most of the variations seem pointless, but I like the cobalt one as a basis for a dark theme.  I plan to improve it, using current Idle (Classic and New seem the same, so why two?), with my changes.  I will upload when I have something I like.

The change I made is to color the backgrounds of 'shell', 'stdout', and 'stderr' light yellow, light blue, and light red.  These changes work so well for me, at least, that I have thought of adding them to an 'Idle New' that is really new and different.  But transitions are hard.  The main other thing I don't like in 'Idle ???' is the orange for keywords, as too 'different' from the purple and blue of builtings and defined. But green is semi standard for strings and that does not leave room for three colors on the blue side.