The latest release of MacOS 10.14 introduce a Dark mode. It does look great and make it really easy on the eye at night. Personally though, I do not like it during the day. Especially for apps that have presents lots of information such as DevonThink Pro or even Mail. In broad day light, I find the light mode easier.
There is an App that switch between the two for you automatically: NightOwl. You have three options (as shown below): manually from the menu icon, based on sunrise and sunset (from your location) and scheduled. As you can see it is actually set on Sunrise/Sunset time. The top most manual settings become available if you do not select one of the two automated options.
Light, well done, super easy to use and it works! I like it so much that I donate to the author, Benjamin Kramser. Hopefully, he will continue to support it (or Apple will pick it up in the future).