As the title suggest I really like the looks of geary and the smaller memory footprint vs thunderbird but where is the dark theme?
Geary 0.12 and later uses whichever GTK+3 theme your desktop environment has been configured to use, so if you want Geary to use a dark theme you'll need to configure your desktop's settings to enable that.
There is some support for customising the CSS used to display individual messages in a conversation, see the Geary FAQ for details. This does not currently apply to the Composer at the moment, but there is an open ticket requesting that.
Thanks to Jaraj Fiala for this answer:
sure, you can either change your GTK+ theme to a dark theme like Adwaita-dark, or launch Geary with:
env GTK_THEME=Adwaita-dark org.gnome.Geary
in my case with Linux Mint Cinnamon 18.3 I use (
env GTK_THEME=Adwaita-dark geary )
or, if you are using the Flatpak version (can recommend):
flatpak run --env=GTK_THEME=Adwaita-dark org.gnome.Geary
turns out you can set the GTK_THEME for other apps as well using the env flag!!