Make fold highlight respect colorscheme

I'm not skilled on highlight compatibility so I put this nasty lines:

https://github.com/albfan/ag.vim/blob/master/syntax/ag_grp.vim#L93
https://github.com/albfan/ag.vim/blob/master/syntax/ag_grp.vim#L94

for a dark fold highlight, but I'm sure it can be tweaked to work only on aggroup buffer or respect different colorscheme setup globally.

Help required
