custom background color?

[johnmathews]

Describe the solution you'd like
I'd like to set a background color for the parentheses to make them more obbvious. maybe white, which would contrast with the normal background.

is this possible?

Describe alternatives you've considered
I've looked at the config options, but i might be misunderstanding it. my vim script knowledge isnt great. I've looked at #45, but couldnt see a solution there.

[luochen1990]

This is very similar to this commit, excepting this is talking about guibg / ctermbg instead of gui and cterm.

Looking forward for a PR :)