There are three actions to get Flatpak Evolution in full dark mode.
flatpak override --user --env=GTK_THEME=Adwaita:dark org.gnome.Evolution
flatpak run --command=bash org.gnome.Evolution -c "gsettings set org.gnome.evolution.mail preview-unset-html-colors true"
or
Preferences >> Mail Preferences >> HTML Messages >> "Unset colors provided in HTML mails in message preview"
Preferences >> Composer Preferences >> "Inherit theme colors in HTML format"