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.