Interesting… Forced Dark Mode on Android 10 (Developer Option) resulted in a semi-dark mode for Monzo which, while indeed dark , didn’t have a nice visual appearance (work needed on certain sections/headers). See here for an example;
Now on release Android 11 and/or Monzo app v3.53.0, even with the same dev option enabled, Monzo does not go dark. So it seems there has been work in the background to play nicely with the system dark mode, as now it can’t be ‘forced’ into it.