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.
With GTK default theme should set the mode. You even have tools that change the theme from light to dark version during the night. But for some reason it doesn't work with evolution build for debian and I don't feel like fixing it.
Turns out Geary was removed from Debian packages because of some stupid unmet dependency. But hey, it's not my first rodeo. I built it from source. Had to install some dependencies and hack the meson build script a bit but it worked. Aaaaaand... no dark mode. Looks like all the new Gnome tools only work well in gnome. Geary also doesn't have a tray icon, it only supports the Gnome widget or whatever it's called.
Thunderbird with gnome theme from same developer of Firefox theme for gnome is fantastic. However, I use in my desktop evolution without problems when switching to dark theme.
Finally I forgave Thunderbird and went back to it. There's birdtray app available in packages that handles the tray icon notifications and works pretty well.
Evolution can be forced to go to dark mode. I did it on my desktop with a script I found online. And, of course I am on my laptop and can't find it anywhere right now.
When I get back on my desktop I'll go through my bash history and find it.