Vim doesn’t have a native way of detecting system appearance yet, but you can do it yourself. Here’s how.

macOS

On macOS you can add the following lines to your .vimrc to check the system appearance every 2s and change the theme accordingly:

" Automatically switch light/dark background + theme
function! SetAppearance(...)
  try
    let s:mode = systemlist("defaults read -g AppleInterfaceStyle")[0]
    let s:new_bg = ""
    let s:new_theme = ""
    if s:mode ==? "Dark"
      let s:new_bg = "dark"
      let s:new_theme = "base16-unikitty-dark"
    else
      let s:new_bg = "light"
      let s:new_theme = "base16-unikitty-light"
    endif
    if &background !=? s:new_bg
      let &background = s:new_bg
      execute 'colorscheme '.s:new_theme
    endif
  catch
    " Ignore errors
  endtry
endfunction
call SetAppearance()
call timer_start(2000, "SetAppearance", {"repeat": -1})

You probably want to replace base16-unikitty-{dark,light} with your theme. If two seconds is a too long delay for you, change the 2000 in the last line to something else (a low number might impact performance).