turtledemo imports IDLE's Percolator, ColorDelegator, and read-only text viewer and uses the first two to colorize demo text it loads into the text viewer.  This works fine for traditional dark on white text color themes but not for IDLE's new white, etc, on dark theme (IDLE Dark).  The reason is that the text viewer was designed to display black on white text, not colorized  code.  IDLE's editor tells tk how to display normal text.  The text view does not, so normal text is the default black on white.  The result is that special items are color on 'black' (dark blue actually) while normal text is black on white.  The strange result can be viewed by selecting IDLE Dark in the preferences dialog and then selecting Help - Turtledemo or running 'python -m turtledemo' in a console.

The immediate workaround is for anyone running turtledemo to use a normal color theme, at least temporarily.  A longer-term solution might be to add a startup option to the text viewer to use the 'normal text' colors of the current theme.