title: Tkinter ttk Checkbutton check not showing with style
body: <p>I'm working on my GUI with python ttk; and I have this problem where the "check" on my ttk checkbuttons won't show with my style, so there's no way to know if it's activated or not. I've tried changing different style options, but I can't find any such option for the "default" theme.</p>
  
  <p>I do <strong>not</strong> want to change my theme as that would ruin my other styles.</p>
  
  <p>How can I change the makeup and layout of a ttk checkbutton?</p>
  
  <pre class="lang-py prettyprint-override"><code>from tkinter import ttk
  tk = Tk()
  style = ttk.Style()
  style.theme_use('default')
  ttk.Checkbutton(tk, text='hello there').grid()
  </code></pre>