vim - dim inactive split panes

In neovim(v0.2.1), the following configuration will dim inactive panes:

hi ActiveWindow ctermbg=16 | hi InactiveWindow ctermbg=233
set winhighlight=Normal:ActiveWindow,NormalNC:InactiveWindow

Changing the background colour as you describe would require different colourschemes for different Vim windows. As far as I know this is not possible, as it is a global setting (see this answer from a few days ago).

As a visual aid to which window is active, I find the statusline is usally sufficient. The highlight groups are different for the active window (hi StatusLine) and any innactive windows (hi StatusLineNC). You could either choose a colourscheme with a very stark constrast between them, or edit your favourite colourscheme.

Throwing this out there as a new answer. https://github.com/TaDaa/vimade fades inactive buffers, preserves syntax highlighting, and can also fade signs. I am the author, but I figure this might be useful to some as a newer alternative. Supports nvim and vim8, 256 color terminals, termguicolors, and gui.

I have come up with the following solution (using 'colorcolumn' and unsetting 'cursorline'):

" Dim inactive windows using 'colorcolumn' setting
" This tends to slow down redrawing, but is very useful.
" Based on https://groups.google.com/d/msg/vim_use/IJU-Vk-QLJE/xz4hjPjCRBUJ
" XXX: this will only work with lines containing text (i.e. not '~')
function! s:DimInactiveWindows()
  for i in range(1, tabpagewinnr(tabpagenr(), '$'))
    let l:range = ""
    if i != winnr()
      if &wrap
        " HACK: when wrapping lines is enabled, we use the maximum number
        " of columns getting highlighted. This might get calculated by
        " looking for the longest visible line and using a multiple of
        " winwidth().
        let l:width=256 " max
      else
        let l:width=winwidth(i)
      endif
      let l:range = join(range(1, l:width), ',')
    endif
    call setwinvar(i, '&colorcolumn', l:range)
  endfor
endfunction
augroup DimInactiveWindows
  au!
  au WinEnter * call s:DimInactiveWindows()
  au WinEnter * set cursorline
  au WinLeave * set nocursorline
augroup END

View it at my (current) dotfiles: https://github.com/blueyed/dotfiles/blob/master/vimrc#L351

Update I have created a plugin out of it: https://github.com/blueyed/vim-diminactive