I updated to Mohave and did *not* switch to new dark theme.]
I installed 64 bit 3.7.2rc1.  Start IDLE.

import tkinter
root = tkinter.Tk()

Black on white Tk window appears.  Move mouse until resize arrow appears. Left click.  Move mouse.  Window turns uniformly black.  Ugh.  Cannot see contents that am trying to resize to.  Release button.  Background returns to white.  Same thing happens with Text added, and hence with IDLE windows and such dialogs as can be resized.