Check out how to do automatic website “dark mode” in Webflow compatible with Safari Technology Preview 68 and MacOS Mojave 10.14.
https://webflow.com/website/CSS-Color-Scheme-prefers-dark?rfsn=1238407.36b81
Then follow then link to vote for future support of this CSS media query inside the Webflow Designer.
https://wishlist.webflow.com/ideas/WEBFLOW-I-1553