This idea involves two steps, the first is on iOS developers and the second is on the user.
This proposal requires that each developer should create a Siri Intent Shortcut (the shortcuts that can run in the background) to turn on Dark Mode and turn on Light Mode. Or, if the developer offers multiple themes just provide a Shortcut for each theme style. On app start up, the developer should donate these Shortcuts via INVoiceShortcutCenter.shared.setShortcutSuggestions. This will allow the shortcuts to be accessed via the Shortcuts app regardless of if a theme has been activated. Of course, a developer should also donate these shortcuts whenever the user changes the theme manually so the system learns user preferences. The next version of Indigenous will be adding these shortcuts as well.
Now that each iOS app that contains a Dark Mode or Theme theoretically makes that available as a background enabled Siri Shortcut, a user can create two multi-step Shortcuts: a Dark Mode and a Light Mode. The user would drag all the Dark/Light mode Shortcuts into the multi-step Shortcut and could even add other things such as modifying the system Brightness up or down.
With the above a user could then switch between Dark and Light mode in iOS via a Siri voice command, an icon on the Homescreen or the Shortcuts widget.
Apple may support system-wide Dark Mode in iOS 13 or 14 but why wait when Apple has provided the tools to enable this for a large majority of apps and users today?