The only thing missing is the OS, which I presume is some version and distribution of Linux.

IDLE used tkinter which uses tcl/tk.  From what you say, the latter, at least on Linux, picks up its default colors from the window manager, which for you happens to be KDE.  (This seems not true, at least not yet, on current macOS, as switching to the new dark theme has no effect on IDLE.)  This suggests that when a widget has both foreground and background settings, IDLE should leave both alone to use compatible defaults or set both.

Except for text windows (which have both fore- and back-grounds) and frames and canvases (which only have background), IDLE generally does not set colors.  I just verified this for the settings and search dialogs.  Do these switch to light on dark when you set you screen to the same?

Setting autocomplete bg to white is an anomaly from before 2005.  It is useless or wrong and I think we should just delete the setting.

Setting the calltip and tooltips backgrounds (tooltip.py, line 162) to yellow is intentional, to contrast with the text.  So we should also set the foregrounds (to black).  I don't think these need to be configurable.

Cheryl, does your Ubuntu desktop have a dark theme option?  If so, does it affect tk defaults and hence IDLE?