Made by instance-id, https://gist.github.com/instance-id/3046ad29bd8782a3b3398cd9ae48ca92
I just packaged it to try out Unity's new package system and hosting packages on github.
Thanks to TheZombieKiller and Peter77 for creating this
https://forum.unity.com/threads/editor-skinning-thread.711059/#post-4785434
Tested on Unity 2019.3.0b1 - 95% Dark mode Conversion
Example Screenshot - https://i.imgur.com/9q5VPQk.png
(Note - Once I ran this, I had to hit play and then it took effect)