title: Swift Cocoa manipulate design of titlebar buttons
body: <p>I'm currently developing a Swift/Cocoa application with XCode 12.3 and Swift 5.</p>
  <p>I'd like to manipulate the design of the close button in the titlebar of a NSWindow.</p>
  <p>I've already tried to change the design using</p>
  <pre><code>self.window?.standardWindowButton(.closeButton)?.layer. ... = ...
  </code></pre>
  <p>But this approach seems not to be working - at least for me. Do you have any ideas on how to make it possible?</p>