title: Changing an NSWindow border color
body: <p>I need to change the border color around my app's NSWindow. </p>
  
  <p>Does anyone know where the window border gets drawn, and how do I change the color/override the method to draw the border? </p>
  
  <p>I've noticed that Tweetbot does this:</p>
  
  <p><img src="https://i.stack.imgur.com/sNHl5.jpg" alt="Regular border vs Tweetbot border"></p>