A fork to try and add system setting dynamic dark mode.
Build and run the "Nightshift Extension" target in Xcode. When prompted "Choose app to run", select Safari.
In Safari, enable the developer menu at -> Preferences -> Extended -> enable "Show Develop menu in menu bar".
In menu "Develop", enable "Allow Unsigned Extensions".
Enable the extension in Safari at -> Preferences -> Extensions -> enable the checkbox next to "Nighshift" in the left column.
For more information, see Building a Safari App Extension