title: How to reset the Border color of a text box to the default system color
body: <p>I'm changing the color of a textbox to red for validation in WPF</p>
  
  <p>I can't find a way to change it back to the system default upon clearing my form.</p>
  
  <p>I thought there was an easy way to change it back to the default by clearing the border brush proper but I can't seem to find it.</p>
  
  <p>Is there an easy way to reset it to the default with out having to set it manually to the system color?</p>