Unfortunately, CSS custom properties are seemingly not (yet?) supported so the color is just changing when the color mode of the operating system is changing. It was fun to see this work, but I wasn't too happy since it would not always match the chosen color theme on my site.
Im pretty happy with that solution for the moment, although I'm a bit concerned that I might forget to change the color in these two additional places when I'm fiddling around in the stylesheets next time. Maybe this is the right point to think about design tokens. I like this small detail and I'm curious if other browser vendors will provide something similar in the future. People have for example also used it to make notifications more obvious.
A lot of the new features of Safari in the upcoming release of macOS are shown in this video on apple.com.