Missing "BG" color for `cdeckInvisible` highlight

[AndrewRadev]

It seems like on my particular colorscheme, the syntax file errors out with:

E420: BG color unknown

It happens on this line:

hi cdeckInvisible guifg=bg guibg=bg ctermbg=bg ctermfg=bg

The problem, I think, is that the "bg" color only works if the Normal syntax group has a set background. In my case (a custom colorscheme), it looks like this:

hi Normal cterm=NONE ctermfg=White
hi Normal guifg=#dddddd guibg=Black

Even if I set ctermbg=NONE, it's the same issue -- it seems like it only works if there's a concrete color set, or I might be doing something wrong. The reason I'm not setting a specific color is because I'm using transparency in my terminal, so I'd like to keep this functionality.

Unfortunately, I'm not sure what a solution might look like. It would be nice if there was a way to check if "bg" was defined. This could work as a best-effort kind of thing, maybe:

try
  hi cdeckInvisible guifg=bg guibg=bg ctermbg=bg ctermfg=bg
catch /E420:/
  " No "bg" set, let's try our best to hide it:
  if &background == 'dark'
    hi cdeckInvisible guifg=Black guibg=Black ctermbg=Black ctermfg=Black
  elseif &background == 'light'
    hi cdeckInvisible guifg=White guibg=White ctermbg=White ctermfg=White
  endif
endtry

I've attached #3 as a PR that implements this fix.