The whole concept of having to do dark mode on a per-app basis seems bizarre. Like, IMHO, this should be something that GTK or Qt handle systemwide. Yeah, maybe for a few apps they need a little work to adapt to dark mode, like if they generate images or something, but...
I use mu4e in emacs in a terminal, and my terminal is light-on-dark, but I imagine that that's probably not what you're looking for.