Set VIM theme depending on OS theme mode. Checks OSX value `AppleInterfaceStyle` and acts accordingly.

set_theme.vim

" COLORS AND THEMING ##########################################################
function! SetBackgroundMode(...)
     let g:new_bg = "light"
     let g:ayucolor = "light" # If your theme supports light mode
     let s:mode = systemlist("defaults read -g AppleInterfaceStyle")[0]

     if s:mode ==? "Dark"
         let g:ayucolor = "dark" # If your theme supports dark mode
         let g:new_bg = "dark"
     endif

     if &background !=? g:new_bg
         let &background = g:new_bg
     endif

     colorscheme ayu #Set your theme here
     hi Normal guibg=NONE ctermbg=NONE
endfunction
call SetBackgroundMode()
call timer_start(30000,"SetBackgroundMode", {"repeat": -1})