I use Carrot Weather, which for me is one of the cutting edge weather apps on iOS. They have a notification centre widget, where you can turn dark mode on or off. I’m not sure if this is known or not, but to me the background for the Monzo widget doesn’t tie in with any of the stock app widgets.
I will add screenshots once I’m home as I think that will highlight it more. Maybe I’m being too picky. The option to switch between dark and light would be nice too, but not essential.