Since the release of Safari Technology Preview 68 in October, Apple has been building support for the use of ‌Dark Mode‌ CSS, which can be enabled via its Develop -> Experimental Features menu.

Values for most CSS variables are assigned via Sass variables; customization via CSS and Sass are both supported. To make it easier to modify custom components, investments have been made in ...

CSS-Tricks.com has an article listing some of the customizations you may wish to consider.For example, there’s a way to make images darker with CSS so that they display better in dark mode.