oh-my-fish/functions/Theme.fish

function Theme --argument-names name
  set -g fish_theme $name

  if [ -e $fish_path/themes/$name -o -e $fish_custom/themes/$name ]
    import themes/$name
  else
    set_color red
    echo "Theme '$name' is not installed. Run 'fish install' to download and install it."
    set_color normal
  end
end