(Neo)vim FocusGained not working as expected

Question

I've setup the Base16 color theme, including Base16-shell and Base16-vim, ( for my terminal (iTerm2) and Neovim. So that I can change themes from the command line I created a bash function that lets me say color <theme> and the base16-shell script for that theme is executed, and the terminal colors changed. This bash function also updates a file, .nvim_background, which is sourced by my Neovim configuration, ~/.config/nvim/init.vim. I would like the colors to change in any open Neovim session when that session gains focus after the color change.

Here is the .nvim_background file:

if !exists('g:colors_name') || g:colors_name != 'base16-chalk'
  colorscheme base16-chalk
endif

And the .nvim_template used to modify it:

if !exists('g:colors_name') || g:colors_name != 'placeholder'
  colorscheme placeholder
endif

Here is the function and augroup from my Neovim init.vim file:

" Base16 setup
function s:CheckColorScheme()

  colorscheme base16-default-dark

  if filereadable("~/.config/nvim/.nvim_background")
    let base16colorspace=256
    source ~/.config/nvim/.nvim_background
  endif
endfunction

" if v:progname !=# 'vi'

  if has('autocmd')
    augroup MyAutocolor
      autocmd!
      autocmd FocusGained * call s:CheckColorScheme()
    augroup END
  endif

"   call s:CheckColorScheme()

" endif

The bash function causes the color change in the terminal to happen immediately, and I can see that the .nvim_background file is getting updated with the new theme name. But when I move focus back to Neovim, the colors do not change. I can do :color base16-<theme> and change the colors in Neovim, so I know the base16 theme plugin is properly installed.

How do I setup the augroup so that it fires when a FocusGained event happens? Or maybe a better question is, how do I setup the s:CheckColorScheme function so that it is called anytime Neovim regains focus?

For completeness sake, here is the bash function that manipulates the colors:

# Base16 helpers {{{
#
# set -x

BASE16_DIR=~/.config/base16-shell/scripts
BASE16_CONFIG=~/.config/.base16
BASE16_NVIM_TEMPLATE=~/.config/nvim/.nvim_template
BASE16_NVIM=~/.config/nvim/.nvim_background
BASE16_DEFAULT="base16-material-darker.sh"

color() {
  SCHEME="$1"

  if [ $# -eq 0 -a -s "$BASE16_CONFIG" ]; then
    cat ~/.config/.base16
    return
  fi

  if [[ "$SCHEME" = 'help' ]]; then
    ACTION='help'
  elif [[ "$SCHEME" = 'default' ]]; then
    ACTION='default'
  else
    ACTION='set'
  fi

  case "$ACTION" in
  set)
    if [[ "$SCHEME" = 'default' ]]; then
      FILE="$BASE16_DIR/base16-material-darker.sh"
      NVIM='base16-material-darker'
    else
      FILE="$BASE16_DIR/base16-$SCHEME.sh"
      NVIM="base16-$SCHEME"
    fi

    if [[ -x "$FILE" ]]; then
      echo "$SCHEME" >| "$BASE16_CONFIG"
      "$FILE"
      sed "s/placeholder/$NVIM/g" "$BASE16_NVIM_TEMPLATE" >| "$BASE16_NVIM"
    else
      echo "Scheme '$SCHEME' not found in $BASE16_DIR"
      return 1
    fi
    ;;
  default)
    FILE="$BASE16_DIR/$BASE16_DEFAULT"
    if [[ -x "$FILE" ]]; then
      echo "$BASE16_DEFAULT" >! "$BASE16_CONFIG"
      "$FILE"
    fi
    ;;
  help)
    echo 'color set [ocean|grayscale|material-darker|default|...]'
    echo
    echo 'Available schemes:'
    find ~/.config/base16-shell -name 'base16-*.sh' | \
      sed -E 's|.+/base16-||' | \
      sort | \
      column
      ;;

  *)
    echo 'Unknown subcommand: use one of {default,set,help}'
    ;;
  esac

}

# }}}