We can use an if-statement to set `bg=dark` if it’s currently “light”, and vice versa:

:if &bg == "light" | set bg=dark | endif

:if &bg == "dark" | set bg=light | endif

Combining these, we can set our background color based on the `AppleInterfaceStyle`:

:if system("defaults read -g AppleInterfaceStyle") == "Dark\n"
: if &bg == "light" | set bg=dark | endif
:else
: if &bg == "dark" | set bg=light | endif
:endif

With our prototype done, let’s build that plugin. After settling on a name, we’ll create a directory and a file to store our plugin in.

$ mkdir -p ~/.vim/pack/plugins/start/vim-nightfall/plugin
$ nvim ~/.vim/pack/plugins/start/vim-nightfall/plugin/nightfall.vim

In nightfall.vim, we'll define a function:

function UpdateBackground()
if system("defaults read -g AppleInterfaceStyle") == "Dark\n"
if &bg == "light" | set bg=dark | endif
else
if &bg == "dark" | set bg=light | endif
endif
endfunction

Let’s try our function. To load our file, call `:source %` (`%` is a shortcut for the current file).

Then, we can switch the background by running `:call UpdateBackground()`. It should switch to match macOS’s background mode. 🎉

We’ll use autocommands, which fire automatically when a certain trigger happens. In our case, we’ll call our function when Vim gains focus (`FocusGained`) or when entering a buffer (`BufEnter`):

:autocmd FocusGained,BufEnter * call UpdateBackground()

To add our autocommand to our plugin, we’ll wrap it in an `augroup` to make sure it’s reloaded properly when we load the plugin multiple times, for whatever reason:

augroup nightfall
autocmd!
autocmd FocusGained,BufEnter * call UpdateBackground()
augroup END

That’s it! Sourcing our plugin once more, we can switch macOS’s appearance to dark mode and have Vim automatically follow when it gains focus.

$ git push origin main

https://github.com/jeffkreeftmeijer/vim-nightfall 🖐🎤

That’s it for now! Please take it for a spin and let me know if you find something I can improve on.

Have a smart configuration in your ~/.vimrc yourself? Turn it into a plugin! A good place to start (and the source of most of my VimL-fu) is https://learnvimscriptthehardway.stevelosh.com.