title: How to set the background color of a ttk.Combobox
body: <p>I have a problem to set background color for Combobox using tkinter ttk with 'vista' theme (I'm using Python 3). I've tried code from here <a href="https://stackoverflow.com/questions/18610519/ttk-combobox-glitch-when-state-is-read-only-and-out-of-focus">ttk.Combobox glitch when state is read-only and out of focus</a> </p>
  
  <pre><code>import tkinter as tk
  from tkinter import ttk
  
  root = tk.Tk()
  combo = ttk.Combobox(root, values=['1', '2', '3'])
  combo['state'] = 'readonly'
  combo.pack()
  tk.Entry(root).pack()
  
  style = ttk.Style()
  style.map('TCombobox', selectbackground=[('readonly', 'red')])
  #style.map('TCombobox', fieldbackground=[('readonly', 'blue')]) #not working as well
  </code></pre>
  
  <p>But this will change only background for text, rest part of combobox rests white. Also I saw a post on the tcl forum: <a href="http://wiki.tcl.tk/15780" rel="noreferrer">http://wiki.tcl.tk/15780</a> and I've tried with 'fieldbackground' but it seems that tkinter ignores this parameter. Do you have any idea how to solve it? Maybe there is a way to configure only specific style in specific theme? I saw that for 'default' theme, the background changes to gray color if state is 'readonly'.</p>