tkinter (ttk) combobox dropdown text is white on white

[Jim Lee]

Given the following snippet (Python 3.6.5, Tk 8.6.8):

import tkinter as tk
from tkinter import ttk

root = tk.Tk()
cb = ttk.Combobox(root)
cb.grid(row=0, column=0, sticky='NSEW')
cb['values'] = ['one', 'two', 'three', 'four']
root.mainloop()

The text of the values in the combobox dropdown list is white on 
white.   The *selected* item in the list is white on light grey, but all 
the unselected items are invisible (white on white).  I have played with 
themes and styles, and can alter the foreground and background colors of 
the combobox itself, but I cannot figure out how to alter the appearance 
of entries in the dropdown list. Any pointers?


Thanks,

-Jim Lee