There is no setting in the app itself, mostly because Apple provides a convenient way to toggle this on and off.

Apple has instructions here on how to do it, including adding it to "Control Center".   (Control Center is the screen that can you swipe down anytime and has shortcut buttons on it for things like flashlight).

https://support.apple.com/en-us/108350

People have also set up other ways to do this, like "tap on the back of the phone". If you search for:

toggle dark mode ios

you'll find lots of articles about this topic.  Maybe we should add this to the app, but hopefully this will be convenient enough, and then it also works for other apps apart from ours.