Doesn't work with slate colorscheme

I am using the slate colorscheme.

I set `g:limelight_conceal_ctermfg` just like in the README. However, when it gets to like 139 and 140 in `autoload/limelight.vim`, `synIDattr` returns an empty string for both `fg` and `bg`.

So then when it gets down to likes 165 and 166, calling the function `s:gray_contiguous` on the empty string leads to throw the unsupported error.

I am not sure if if it supposed to be in the guicolors section, but it would still throw the error at line 146 if so.

Not sure why slate doesn't return the fg or bg.