Most applications (Google Chrome, Nautilus, Skype, Terminal) are Dark but a few are not. Probably a mismatch in GTK ressources. Claws Mail is critical for me because I use it very often.
The adw-gtk3 package is now called adw-gtk-theme which replaces the old adw-gtk-theme. I know it’s a little confusing.
With that change, you may need to set Legacy Applications to Adw-gtk3-dark in the Tweaks application under Appearance. It may have been set to the now removed Adw-dark previously.