Unite 4 not only brings support for dark mode in the creation tool, but also to Unite apps themselves. Unite by default automatically detects the system setting, and not only does it convert all Unite elements to appropriately match, but will also match supported sites within the app. For example, a user using YouTube who switches into dark mode will see his YouTube theme switch as well.
Unite apps also allow you to change their mode despite the system setting by visiting Your app name in the menu bar → Preferences (⌘ ,) → Appearance. This could be useful for users who wish to use most of their sites in one mode but have a particular site they want to stay in the other.