MacOS Mojave introduced Dark Mode, which allows MacOS users to enable a darker interface. According to Apple:

In Dark Mode, the system adopts a darker color palette for all windows, views, menus, and controls. The system also uses more vibrancy to make foreground content stand out against the darker backgrounds.

In Safari Technology Preview 68, they’ve also added support for websites to add support for Dark Mode with a new media query:

@media (prefers-color-scheme: dark)

I’ve added support to my theme, which now looks like this in the latest Safari Technology Preview if you’ve got Dark Mode enabled: