If any other olds are having the same problem, go to your settings in the phone, under display and brightness, then click light or dark. It's at the top.
Just adding my support to the feature to make it an app setting vs defaulting to iOS.
I don't want to turn night mode off on my phone as I like it in most cases, but I just don't like viewing TexAgs in night mode - some of the UI elements are too dark and there's too much contrast with white text on an almost black background.
This is literally the one thing that keeps me from using the app as my primary means for accessing TexAgs.