It will use whatever `bat` theme is set. Bat itself doesn't do system theme detection, but it's easy enough to do in a script - I have one that gets called when my system theme changes where I set themes for various programs. Looks like this for dark mode:

    #!/usr/bin/fish
    gsettings set org.gnome.desktop.interface gtk-theme "Adwaita-dark"
    fish_config theme choose "ayu Mirage"
    yes | fish_config theme save
    echo "--theme=OneHalfDark" > ~/.config/bat/config
    kitten themes --reload-in=all "Ayu Mirage"

The bat config change will make delta respect my "system" theme.