Install gnome-themes, it contains adwaita-dark:
sudo pacman -S gnome-themes-extra
From the AUR install adwaita-qt5 and adwaita-qt6 so qt apps can pick up the adwaita-dark theme.
Set the following env vars to apply the themes:
GTK_THEME=Adwaita:dark
GTK2_RC_FILES=/usr/share/themes/Adwaita-dark/gtk-2.0/gtkrc
QT_STYLE_OVERRIDE=adwaita-dark