Problem as I understand it: various tk widgets and Idle popups and dialogs mix, for instance, a hard-coded background (white) with a sometimes incompatible (no or low contrast) system-default foreground, which is also white when the system background is black.  Hence the sad screenshot for #16984: https://bugs.python.org/file28755/snapshot-idle.png.

Some of this may be tk's fault for not anticipating reversed graphics screen, some may be Idle's (I doubt tkinter gets involved).  In any case, a solution is for Idle to set the colors for *every* widget it creates.  The colors could be fixed in particular cases, but we could instead use one of the currently 13 user settings*.  #24820 is about adding a light on dark theme to Idle.  When we start using ttk widgets (within a month), I believe this will be possible by setting an explicit default color pair for each class.

* I am thinking I might try using the new dark theme on my white-background system, but I cannot know that I would want all the dialogs to use it.  We might want to allow separate tagged text themes (what the current highlights are for) and widget themes.  This will take some experimentation.