macOS 10.14.4 will bring automatic Dark Mode to Safari. This means that if a website offers a dark theme, it will automatically be enabled (via iDownloadBlog).
Safari Preview 68
In Safari Technology Preview 68 Apple introduced a Dark Mode API, which you can find in Safari’s Develop > Experimental Features > Dark Mode CSS Support menu. But right now it’s only available in the macOS 10.14.4 developer beta which was released last week.
To turn on Dark Mode go to Preferences > General > Appearance. Now, all we need is automatic Dark Mode.