CSS Color Scheme: prefers (dark)
With Safari Technology Preview 68 and MacOS 10.14, it is now possibly for the browser to recognize if the operating system is configured to dark mode and adjust the css as defined by the @media query. Soon this will become supported by all major browsers and operating systems.
Try it out and see how it's done.
Then vote for Webflow to support this in the Designer. https://wishlist.webflow.com/ideas/WEBFLOW-I-1553