Dim viewport into VM display using black instead white when OS is in dark mode

[JakeWharton]

In the MacOS app, the viewport into the VM's display is slightly dimmed with a translucent overlay toward white.

When the OS is in dark mode, white is still used as the target which creates a slightly jarring bright section of the UI.

If black was used instead, this would blend in more with the dark theme. Here's a 10-second mockup from Preview.app showing what I mean: