Safari 12 that shipped with Mojave does not have a way to detect whether a user has a dark mode or not.
The good news is: Safari Tech Preview 68 supports Dark Mode! And Safari 12.1 might support it in a few months too. The CSS itself is very simple.
As soon as Safari supports this media query, I’m switching Drinking Caffeine to using this as the determinant for whether you get light mode or dark mode. Until then, you can adjust it at the settings page.