There is a separate Dark Mode stack that applies some default dark mode stylings globally on the page. The Dark Mode Vis stack can be used alongside the Dark Mode stack (or completely on its own) to conditionally show different content to people that visit your webpage and have a personal preference for viewing in Dark Mode (or Light Mode).
When you add this stack to the page you will be presented with 2 drop zones - one for people that either have no preference or have a preference for Light Mode, and another for those that have a preference for Dark Mode. You simply add and style whatever stacks you want to into each of these drop zones.
When someone visits your page they will be shown one piece of content or the other (depending on their preference).