title: Strange background appearance in tkk.Frame
body: <p>I discovered an anomaly in tkinter. I have attached my script below to show the anomaly.</p>
  
  <p>Running this script, I found on my system (Ubuntu 16.04.03) that the background of the <code>ttk.Frame</code> appears strange. If I move my mouse pointer in and out of the <code>ttk.Frame</code>, the background appearance can change and appear gibberish (e.g. backgrounds from other background windows may appear, strange color patterns may appear,....).</p>
  
  <p>After some investigations, I discovered that by replacing  <code>background='light purple'</code> with <code>background='white'</code>, the anomaly disappears and the <code>ttk.Frame</code> appearance behaves as was defined. That is, the fault was caused by using a background/colorname that tkinter did not recognised.</p>
  
  <p>Questions:</p>
  
  <ol>
  <li><p>Does this anomaly occur in a Mac, Windows or other Linux systems? </p></li>
  <li><p>What are the valid background names that tkinter accepts? I was not
  able to locate it in tkinter's <code>__init__.py</code> or <code>ttk.py</code>. Where can
  I find them?</p></li>
  <li>I think tkinter requires a more robust background name check routine
  to mitigate this anomaly. Who should I attention this to?</li>
  </ol>
  
  <p><strong>Test Script:</strong></p>
  
  <pre><code>#!/usr/bin/python3
  # -*- coding: utf-8 -*-
  
  import tkinter as tk
  import tkinter.ttk as ttk
  
  
  class App(ttk.Frame):
  
      def __init__(self, parent=None, *args, **kwargs):
  
          style=ttk.Style()
          style.configure('App.TFrame', background='light purple')
  
          ttk.Frame.__init__(self, parent, style='App.TFrame', borderwidth=10,
                             relief='raised', width=200, height=200)
          self.parent = parent
          self.parent.geometry('300x300')
  
  
  if __name__ == '__main__':
      root = tk.Tk()
      app = App(root)
      app.grid(row=0, column=0, sticky='nsew')
      app.grid_propagate(0)
  
      root.mainloop()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/R2Lm4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/R2Lm4.png" alt="StrangettkFrameBackground"></a>  <a href="https://i.stack.imgur.com/huDqH.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/huDqH.png" alt="StrangettkFrameBackground2"></a></p>